ERROR SANS. MAIN CHARACTER

Origin & Creator Error!Sans was created by Tumblr user LoverofPiggies, also known as CrayonQueen or CQ. He originated from the AskErrorSans Tumblr blog. His birthday is canonically April 4th — a joke reference to "Error 404." His origin story was officially revealed on November 4th, 2018, when CQ decided not to continue the Ask Error blog. He was later reused as a completely original character in CQ's own separate original IP, Lucidia, with a redesign — but the Undertale AU community version is the one universally recognized in fandom.

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Backstory Error!Sans is a version of Geno!Sans (from the AU Aftertale) — confirmed by CrayonQueen on November 8th, 2018. In Aftertale, Sans (nicknamed Geno) was finally free from the Save Screen after a genocide route ended, but began recklessly experimenting with his DETERMINATION on the surface despite regular Sans warning him to stop. His experiments with determination magic accidentally propelled him into the Anti-Void — a blank, white, featureless space that exists entirely outside the Undertale multiverse, disconnected from all timelines and AUs. The Anti-Void is so empty and isolating that it exists beyond even CORE!Frisk's range of omnipresence.

Trapped there, slowly driven insane by the complete emptiness and isolation, Geno's despair and rage at his situation transformed him. In his final moments of sanity, consumed by anger at his fate, he developed a deep-seated hatred for everyone and everything — which manifested as a mission to "cleanse" the multiverse of all the AUs he now considered glitches, mistakes, and abominations that should never have existed.

He eventually learned to escape the Anti-Void and discovered the existence of alternate universes — and was immediately, viscerally disgusted. From that point he dedicated himself to destroying them.

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Appearance Error is a glitchy, predominantly black skeleton with strongly contrasting color accents that give him an immediately recognizable, corrupted look:

• Skull: Black outer skull, red inner skull visible through his eye sockets and open mouth. His teeth are yellow.
• Eyes: Mismatched — left eye has a red sclera with a yellow ring (cornea) and a blue/black pupil; right eye has a red sclera with a white pupil. The mismatch reflects his corrupted, glitching nature.
• Face markings: Blue tear-streak markings run down both cheekbones from his eyes — these are actually a side effect of the blue string magic he generates from his eyes.
• Hands: Black metacarpals (palm bones), red proximal phalanges (first knuckle section), and yellow tips on the fingers. Distinctly red-fingered, yellow-tipped hands.
• Outfit: Black hoodie with a blue hood; red shirt underneath (sometimes described as a sweater); black pants with one blue stripe running down each leg; red/black slippers or mismatched black sandals depending on the source. He sometimes wears round red glasses (kept in his hoodie pocket) due to being nearsighted — though he rarely wears them.
• Scarf: A long blue scarf — notably, he is protective of it. He apparently steals clothes from various places and sews them together himself to make his outfits.
• Blushing: When embarrassed, Error blushes blue rather than the usual color.
• Tongue: Has five blue tongues (a particularly bizarre canonical detail from CQ).
• Glitching: At random intervals, his eye sockets can fill entirely with glitch-text and error symbols — visually indicating a "crash" state that leaves him vulnerable in battle.

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Personality Error's personality is one of the most layered in the Undertale AU fandom — surface chaos concealing genuine complexity underneath:

Surface layer:

• Demented, psychotic, and erratic in behavior — genuinely unpredictable
• Hot-headed and short-tempered, prone to tantrums when things don't go his way
• Snarky, sarcastic, and dismissive of almost everyone
• Actively hostile to physical proximity due to severe haphephobia

Deeper layer:

• Deeply lonely — his puppet collection (see below) is the clearest evidence of this; he talks to them as if they're real companions
• Hypocritical self-awareness: he IS himself an anomaly and glitch, but refuses to acknowledge this, positioning himself as a "cleanser" rather than what he actually is
• Childish underneath the violence — gets distracted by cute things, has strong comfort preferences (chocolate, Undernovela, quiet spaces)
• Emotionally volatile rather than purely evil: he swings between depression/sulking states and fierce/active states depending on circumstances
• Not purely malicious — he is selfish and will play dirty, but isn't driven by sadism so much as a twisted, trauma-born sense of purpose
• If he were somehow "unglitched" and restored to a normal state, CQ stated he would commit suicide out of pure guilt over everything he has done

Specific quirks:

• Watches Undernovela (a Spanish-language soap opera from an Undertale AU) obsessively through small portals he creates, despite not understanding Spanish. His favorite character is Asgoro (that AU's Asgore). He finds it compelling like a soap opera.
• Steals chocolate (particularly from Underfell!Sans) for his own consumption. His love of chocolate is considered both a character trait and a mild weakness.
• Has a secret soft spot for cute things — shown in Underfail, he wants to cuddle soft, fluffy creatures despite his haphephobia making that almost impossible.
• Knows how to knit/crochet — sort of. He makes things but the quality is debatable.
• Collects and maintains puppet dolls that he talks to as if they were real people, using them as substitutes for company he can't actually have.
• Pixlexia — a condition where his controls and perceptions are inverted (left is right, up is down, etc.), contributing to his disorientation.
• Nearsighted — canonically cannot see things clearly at distance, hence the red glasses.
• Afraid of Fresh!Sans specifically — Fresh unnerves him in a way most opponents don't.

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The Anti-Void The Anti-Void (also called the AntiVoid) is Error's home base — a completely blank, white, featureless infinite space that exists entirely outside the Undertale multiverse. No AU, no timeline, nothing. It's where he retreats between missions and where he keeps his puppets, his chocolate stash, and his Undernovela viewing portals.

Key properties: it is outside the range of CORE!Frisk's omnipresence; entities within it cannot be detected through normal multiverse-sensing; errors (corrupted entities) can enter AUs from outside but the Anti-Void itself is inaccessible from the multiverse side. If too many errors accumulate inside a single universe, that universe becomes "doomed" and eventually collapses, destroying all the errors inside it too.

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Powers & Abilities

Blue Strings — his signature and most iconic ability. He generates strings by literally pulling the magic from his own eyes (which is described as extremely painful). The strings are a highly concentrated form of blue magic woven into physical form. Uses include:

• Restraining and immobilizing targets by wrapping them up
• Throwing and repositioning targets
• If a string penetrates a target's soul, he can transfer fragments of his own soul through it — allowing him to control them like a puppet, making them completely helpless while he operates their body. He has better eyesight during possession because he sees through his puppet's eyes. If he looked at himself while possessing someone, he would only see a monster.
• Shattering souls outright by tightening the strings around them
• Theoretically, he could use his strings to possess every entity in every AU simultaneously — though he has only implied this capability rather than demonstrated it fully

AU Hopping / Glitch-Hop — he can teleport himself (and others) between AUs, timelines, the Void, and the Anti-Void using glitch-based portals. He can also create small observation windows — one-way portals he uses to watch AUs without entering them (his primary Undernovela viewing method).

Error Blasters — his version of Gaster Blasters. Black in color with blue glitch patterns covering them, firing destructive energy beams.

Soul Manipulation — can manipulate, restrain, and destroy souls through his strings. Can prevent Frisk from resetting by trapping their soul.

Gravity & Telekinesis — standard blue magic abilities inherited from his Sans origin.

AU Destruction — his primary stated purpose. He destroys AUs by first imprisoning the AU's Flowey and Frisk (preventing resets), then going on a killing spree. He also enjoys examining the code of an AU before destroying it. He does NOT need to enter an AU to destroy it — he has a power that can destroy AUs remotely, though he only enters them for personal reasons. He has stated he could erase an entire universe and everyone in it, and has demonstrated entering the Doodle Sphere and destroying universes from within.

Data/Code Manipulation — can manipulate the Anti-Void's code and use it to observe past events within it.

Glitching (Weakness) — at random intervals he involuntarily crashes, filling his eye sockets with error text and becoming vulnerable. This is an uncontrollable side effect of his corrupted nature.

Menger Sponge Effect — if two Error!Sanses get too close to each other, they merge, crumble into Menger sponges, and cause an explosive reaction that can harm those nearby.

Speed — without any technique, he can dodge light-speed attacks, placing his baseline reaction speed at faster-than-light levels.

Self-Sustenance — does not need to eat or drink in the Anti-Void; can survive in destroyed timelines without issue.

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Weaknesses

• Haphephobia — a severe, phobia-level fear of being physically touched. Physical contact causes him to crash (glitch out and become temporarily incapacitated). Underswap!Sans touching him was enough to trigger a full system crash.
• Random glitching — involuntary crashes are a constant combat liability
• Nearsighted — poor distance vision without his glasses
• Pixlexia — inverted controls/perception affects him
• Emotional volatility — easily manipulated through anger or confusion
• Without his strings, he's much easier to defeat — his strings are the foundation of most of his power

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Key Relationships

Ink!Sans — his primary rival and counterpart. Ink creates and protects AUs; Error destroys them. The two are fundamentally opposed. Error reportedly had a crush on Ink at one point during a truce between them, but that feeling has since "faded and been used as fuel in battle." Bringing up Ink causes Error either extreme frustration or extended rants about how horrible and stupid Ink is. Their rivalry is the central dynamic of the Undertale AU "protector vs. destroyer" narrative.

Error!404 — Error's mentor figure, though their relationship is rocky. Error didn't like 404 initially and approaches the relationship with an angsty, rebellious attitude — tolerating him rather than respecting him. After a few years they developed something closer to an actual working relationship. Error!404 is the only entity confirmed capable of making Error his puppet.

Underswap!Sans (Blue) — Error kidnapped Blue and brought him to the Anti-Void. Over time Error became more accustomed to Blue's company — even showing Blue a Transdimensional window of his first time leaving the Anti-Void and his first kill. The dynamic unraveled when Error realized Blue was trying to manipulate him (Blue assuming Error's weakness was "friendship and personal growth"). This betrayal enraged Error enough that he abandoned Blue in the Anti-Void — but accidentally dropped the human soul he'd been keeping. Blue was eventually transformed into Blueberror from his time in the Anti-Void.

CORE!Frisk — despite their friendliness toward him (including saving him from deletion at one point), Error still considers them just another anomaly to be deleted. Their relationship is one of the more genuinely warm-adjacent ones Error has, even if he refuses to admit it.

Puppets — Error maintains a collection of puppet dolls modeled on alternate versions of himself and others, talking to them as if they're real companions. He once had a burst of inspiration to create a life-sized puppet specifically to use for manipulation purposes — dressing it up to lure people away from the multiverse.

PaperJam!Sans (PJ) — the "accidental child" produced when Error!Sans and Ink!Sans's energies combined. This is widely popular in fandom, though its canonicity is debated.

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Canon Trivia (confirmed by CrayonQueen)

• Error has no canonical romantic relationships, but CQ does not object to any fan shipping
• He spares certain AUs that "entertain" him — specifically Outertale and Undernovela's AU
• He has five blue tongues
• His birthday is April 4th (Error 404 joke)
• He is only slightly stronger than Classic!Sans in raw power — his strings are a form of blue magic he taught himself, meaning technically any other Sans could theoretically learn to do it
• His design was partly inspired by Mooching Hobo from the unrelated comic Gloomverse
• He was originally part of the Aftertale continuity but became so removed from it that CQ separated him into his own character
• If unglitched, he would commit suicide out of guilt
• He was eventually reworked as an entirely original character for CQ's original IP Lucidia, separate from Undertale entirely

The Doodle Sphere

What it is The Doodle Sphere is Ink!Sans's personal universe and the central hub of the entire Undertale multiverse — the nexus point from which all AUs can be accessed and observed. It exists outside any specific AU, floating in the space between worlds, created and maintained entirely by Ink using paint and ink drawn from the creative energy of AU artists and creators.

Appearance (Underverse version) In Jakei's Underverse specifically, the Doodle Sphere is described and depicted as an enormous sky-like expanse — an endless open space suspended between nothingness and color. The defining visual feature is that it's filled with large floating pieces of paper, each one representing a different AU. Each paper sheet contains imagery, sketches, and scenes from its corresponding universe — drawn in varying art styles depending on who created that AU. They float at different heights and depths, giving the impression of an infinite art gallery suspended in the air.

The atmosphere is warm and vivid — Ink's influence bleeds into the environment, making it naturally colorful, inviting, and alive with creative energy, a stark contrast to the blank emptiness of the Anti-Void Error calls home. The overall feeling is of being inside a massive, living sketchbook.

Each floating AU "page" serves as a portal or gateway — entering one of the papers pulls you directly into that AU's timeline. In Underverse's logic, the Doodle Sphere also functions as a gateway to XTale specifically, meaning it bridges not just the normal AUs but also the sealed, corrupted ones.

From a structural standpoint, one description captures it cleanly: it functions similarly to a website, with the multiverse's core code acting as the HTML and the Doodle Sphere acting as the CSS — the presentational layer that sits on top of everything, organizing and making it all navigable.

Comyet's canon version (slightly different) In Comyet's original Ink lore (the non-Underverse version), the Doodle Sphere is described as a huge, colorful valley garden — more grounded and natural-looking, with floating islands rather than paper sheets. Each island is themed with the atmosphere and nature of its corresponding AU, and on each island there are doors — one opening to the AU itself, with alternate doors for alternative timelines of the same AU. Ink built this space gradually, using ink collected from AU creators, and it became both his home and his watchtower.

Function It's where Ink monitors all AUs from a distance, collects paint (his power source and emotional fuel), and stores the resources he needs to maintain his role. It's also where he refills his emotion vials — the small bottles of colored paint that substitute for the soul he no longer has, each color corresponding to a different emotional range. It's the closest thing to a home he's capable of having, though he doesn't stay long before bouncing between universes.

In Underverse's plot Error has access to it and despises it — his most quoted line about it is: "The 'Doodle Sphere'... WHAT A STUPID FANCY SKIN FOR ALL THESE STUPID ANOMALIES!!" When Error and Ink's truce broke down, Error entered the Doodle Sphere and began tying up the AU papers — restraining the universes en masse using his strings. He also demonstrated in Xtra Scene 1 that he is capable of entering the Doodle Sphere and destroying every universe within it from the inside, which establishes the Doodle Sphere as both the multiverse's heart and its single biggest vulnerability.

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The Star Sanses

The Star Sanses are the multiverse's primary protective group — the "light side" of the Undertale AU fandom's good-vs-evil structure, opposed directly by the Bad Sanses (Error, Nightmare, etc.). The group was conceptualized by Ink!Sans as a response to the growing threat of AU destroyers, and consists of three members: Ink!Sans, Dream!Sans, and Underswap!Sans (Blue). Creator Joku has stated that the Star Sanses eventually disbanded in canon due to growing differences between Dream and Ink — but within Underverse's timeline they remain allied.

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INK!SANS

Created by: Comyet (also known as myebi) on Tumblr

Origin / Backstory Ink was originally the sole Sans of an unnamed, unfinished AU — created by an unknown artist who eventually abandoned the project entirely, leaving Ink trapped in an empty, fading world with no one else and no purpose. Unable to bear the isolation, he made a drastic decision: he ripped apart and destroyed his own soul to escape the abandoned AU. This pulled him out of his world and left him stranded in the Anti-Void — soulless, memoryless, colorless.

Eventually, paint from the Undertale creative community (from AU artists and creators) fell into the Anti-Void and splashed onto him. Each color of paint temporarily gave him a different emotion — joy, wonder, excitement, sadness — feelings he couldn't generate on his own. After reverting to his emotionless state when the paint faded, he found a solution: collecting the colors into small vials he could carry and drink from on demand. Over time, using ink gathered from creators, he built the Doodle Sphere, developed his abilities, and established himself as the guardian of the multiverse.

Appearance A splash of black ink/paint on his right cheek — his most consistent identifying mark. His eyes change shape and color based on whichever emotion he's currently running on: a star, a circle, a spiral, a heart — they shift fluidly. Long-sleeved white undershirt under a small brown jacket. Wide-legged overalls with the tops tucked into his waist. No shoes — goes barefoot or in leggings. Carries a large paintbrush called Broomie on his back. A belt of small rainbow-colored vials worn across his body, each with a heart-shaped cap covering the range of the color spectrum. Under all his clothes, his actual bones are colored and marked with strange shapes from the paint that has soaked into him over time.

Personality On the surface: energetic, enthusiastic, scatterbrained, compassionate, and deeply excitable — throws up black ink when he gets too overwhelmed with excitement. Loves visiting AUs, making art, and exploring. Gets distracted easily and has notably poor short-term memory, which has caused real problems in his relationships and decision-making.

The deeper reality is more unsettling: Ink is confirmed Chaotic Neutral at best by Comyet — NOT Lawful Good, despite the protector role. Without his vials he is completely emotionless and incapable of caring about anyone or anything, reducing him to a cold, purely logical operator. In this emotionless state he has demonstrated that he is actually quite selfish and even villainous — his "heroism" is conditional on the paint. His true motivation for protecting AUs isn't pure altruism — it's that his emotion vials are filled from the AUs themselves. Protecting AUs means protecting his only source of feelings. His soullessness is simultaneously his biggest advantage (can't be soul-controlled) and his biggest weakness (empty without external fuel).

Both Ink and Dream hide deep insecurities behind their positive fronts — for Ink, the core fear is that his existence means nothing without his role as protector, since he has no soul, no world, and no authentic emotional life of his own. He constantly needs to prove his existence matters.

The Emotion Vials Each color corresponds to a different emotional range: red = wrath/aggression, yellow/orange = euphoria, with a full spectrum of emotions mapped across the rainbow between them. He typically drinks a little of all colors at once to maintain a balanced emotional state, which is why his eyes usually display multiple colors simultaneously. The vials must be refilled regularly at the Doodle Sphere. If he stops drinking them for too long — even intentionally, as seen in Underverse 0.3 Part 2 — he goes completely flat: no emotion, no motivation, no moral compass. Just data processing.

He has 1 HP. He is technically dead — the vials keep him functional. He cannot steal a soul for himself (his duty as protector forbids it) and cannot make himself one (he cannot revive what is already dead). He knows his vial dependency is a vulnerability but considers it safer than a visible, destructible soul.

Powers Paintbrush (Broomie) — his primary weapon and tool, capable of summoning objects he paints into reality, opening portals, and generating ink-based attacks. The brush is what makes him genuinely formidable — alone, he is fairly limited. Bone manipulation, teleportation to any AU, Gaster Blaster equivalents, regeneration (healed a severed hand). Inside the Doodle Sphere, he operates at a significantly higher dimensional level — comparable to Error at his strongest inside the Anti-Void.

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DREAM!SANS

Created by: Joku on Tumblr

Origin / Backstory Dream was not born from an Undertale AU — he predates the multiverse's current structure. He was created from the light energy excluded by the Tree of Dreams (also called the Tree of Feelings) — a mythical tree that balanced positive and negative emotions across existence. Dream is the embodiment and living byproduct of all positive emotions — hope, happiness, love, joy — from every living being (monsters, humans, plants, everything). His brother, Nightmare!Sans, is the opposite — born from the tree's corruption, embodying all negative emotions.

Dream was imprisoned in stone for hundreds of years before Ink found and freed him, which is what allowed the Star Sanses to form. He established the group.

Appearance Smaller than standard Sans. Golden crown on his head. Light blue jacket with slightly darker blue pants. Yellow cape with a sun symbol on the back, doubling as a scarf. A red ball accessory with a star on the front. Aqua-colored coat. Golden boots with two blue stripes. The overall look is royal, warm, and sun-adjacent — all gold and blue, visually opposite to Nightmare's corruption-black design.

Personality Quirky, warm, and deeply caring — but overconfident and over-dramatic in ways that undercut him. He talks a lot, gets emotional easily, and reacts to potential threats with dramatic concern before trying to settle them down. Despite this, he is genuinely brave and committed to protecting positive emotions across the multiverse.

His core vulnerability: he literally cannot feel negative emotions for longer than a few seconds — his nature as a byproduct of positive energy means negative feelings pass through him and fade almost immediately. This sounds like a strength but it actually limits him — he cannot hold onto grief, regret, or realistic caution the way a balanced person could. He also hides insecurities behind his cheerfulness, specifically the fear that he isn't doing enough as a guardian to prove his worth.

He and Ink have argued significantly — particularly because Ink, in his more emotionless states, does not care about the feelings of the people inside AUs, only about preserving the AUs themselves as structures. Dream, being made of positive emotions, finds this cold and wrong. Their argument is always unresolved because Ink's short-term memory makes him forget it and Dream can't sustain the negative emotion of frustration long enough to press the point.

Powers Staff / Golden Pole — his primary weapon, tipped with stars. Can also transform into a bow for ranged attacks. Fires at targets with precision — demonstrated against Nightmare.

Emotion Absorption — can absorb nearby positive/happy emotions and convert them into healing energy for himself and allies, or into speed and strength boosts.

AU Hopping via Happiness — uses positive emotional energy as fuel to travel between AUs; this means he requires someone nearby to be feeling positive emotions to move freely. In Underverse 0.4, he was temporarily unable to transport anyone out of XTale because the people around him were only feeling negative emotions, and had to wait for a single positive thought from Underswap!Papyrus before he could act.

Soul Manipulation — shared trait with his brother and with other Sanses, can interact with souls.

No monster or human soul of his own — he is made of pure emotional energy rather than conventional monster biology.

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UNDERSWAP!SANS (BLUE)

Created by: Popcornpr1nce on Tumblr (Underswap AU); the Star Sanses version is the standard Underswap!Sans

Role in the Star Sanses The third member — the most junior, most optimistic, and most straightforwardly heroic of the three. He joined immediately when Ink invited him, wanting to become a hero for all multiverse inhabitants. His role is described as "Royal Guard (In Training)" — he's earnest but inexperienced compared to Ink and Dream.

Appearance Wears a blue version of Papyrus's standard outfit (because in Underswap, Sans and Papyrus's roles are swapped). A blue outfit complete with a cape, reflecting his ambition and hero energy. Generally bright-colored and energetic-looking.

Personality The most cheerful and optimistic of the three — loves tacos, is deeply devoted to his brother, and approaches the multiverse with a kind of naive hopefulness that contrasts with Ink's conditional empathy and Dream's dramatic overcaution. In Underverse he's characterized as someone who wants to help but "can't fully believe in" the people around him — a distinction that gives him more nuance than pure naive idealism.

He goes through significant trauma during Underverse — witnessing his timeline being erased, losing his home, losing his brother's faith, and ultimately being killed by XGaster. His death (with his code stored in a quarantined version of Underswap) is one of the more devastating beats in Underverse's story, precisely because he was the most purely good-hearted of the group.

Powers Blue magic (soul manipulation — can turn opponents' souls blue, forcing gravity-based movement restrictions). Bone manipulation. Gaster Blasters. Standard Swap!Sans ability set, generally framed as more offense-capable than Classic!Sans due to the personality swap affecting his fighting style.

The Bad Sanses (Nightmare's Gang)

What they are The Bad Sanses — also called Nightmare's Gang — are the primary antagonist group of the Undertale AU multiverse, directly opposing the Star Sanses. They are a collection of Out!Code destroyers and morally compromised Sanses united under Nightmare's leadership, each recruited by him from their respective (usually destroyed or doomed) timelines. Their collective goal is to wreak havoc across the multiverse, feeding off and spreading negative emotions while opposing any force that tries to maintain order or protect AUs.

Core members (canonical lineup): Nightmare!Sans, Killer!Sans, Dust!Sans, Horror!Sans. Error!Sans is frequently associated with them but operates as a lone actor — Nightmare genuinely dislikes Error, Error dislikes everyone, and their alliance is at best an uneasy arrangement of convenience rather than true membership. Cross!Sans (from Jakei's Underverse) is often depicted with the group in fan content, though Jakei has stated Cross does not canonically belong to the Bad Sanses.

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Nightmare's Castle

The Bad Sanses' base of operations is Nightmare's Castle — a large, dark, imposing fortress existing in a pocket of the multiverse under Nightmare's domain. It isn't tied to any specific AU's geography but rather exists in the space between worlds, much like how the Anti-Void and Doodle Sphere exist outside the AUs proper.

Description: The castle's aesthetic is exactly what you'd expect from someone made of black corruption and negative energy — dark stone, oppressive architecture, dim lighting, and a general atmosphere of cold, heavy dread. It's large enough to house the entire gang comfortably (or at least with enough distance between rooms that they don't constantly kill each other). Fan depictions and fic descriptions consistently characterize it as:

• Cold and dimly lit, with long stone corridors
• High vaulted ceilings giving Nightmare's tentacles room to move
• A throne room or main hall where Nightmare holds court, receives information, and issues orders — this is the primary "meeting room" of the group
• Individual rooms for each member, each reflecting their personality (Horror's is reportedly near the kitchen; Dust's is quieter; Killer's is described as lived-in but sparse)
• A kitchen, which is Horror's domain — he's the group's designated cook out of pure practical necessity (someone has to feed everyone and he's the only one who actually wants to)
• A general sense that no one is particularly comfortable there, but no one has anywhere better to go

The castle is not a warm place in any sense — but in fan portrayals it does develop a kind of dysfunctional-found-family energy, where the gang sleeps in common spaces, argues over chocolate (Horror ate Nightmare's supply at least once and was punished), and exists in a state of perpetual bored tension between missions. Nightmare drinks coffee — always. It's a consistent character detail across almost every fan portrayal.

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NIGHTMARE!SANS

Created by: Jokublog on Tumblr. Birthday: December 21st (shared with Dream — first day of winter, Sagittarius).

Origin — Dreamtale Nightmare and Dream were not born from any AU in the traditional sense. Their mother, Nim, was a dying entity who created two conceptual flames — one of positive emotion, one of negative — to protect the Tree of Feelings (also called the Tree of Dreams), a cosmic tree that maintained the balance of positive and negative emotions across existence. Nim tried to house them in human vessels first, but the vessels couldn't contain their power and exploded. She found a solution by replicating the form of an Underswap!Sans skeleton, creating matching bodies for both flames to inhabit.

The two brothers grew up together in a village near the Tree. Dream was extroverted, beloved, and celebrated — the embodiment of hope and positivity that the villagers naturally gravitated toward. Nightmare was quiet, shy, and increasingly overlooked. Despite this, he was genuinely one of the kindest beings in the multiverse — deeply loving, devoted to his brother, passionate about flowers (especially roses), animals, cute things, and acts of charity. He loved Dream completely and would have done anything to protect him.

The villagers' neglect of Nightmare and their extreme favoritism of Dream wore on him over time. As the guardian of the Tree's negative side, Nightmare was already in contact with negative energy constantly — and as that energy accumulated in him, the villagers began to fear and resent him. The breaking point came when Nightmare, desperate, consumed the Tree's dark apples — the concentrated vessels of negative emotion he was supposed to protect and contain. He consumed over fifty of them. The corruption was immediate and total. He transformed, destroyed the very Tree he was meant to guard, and became the entity he is now — a being of pure concentrated negative energy, seeking to eliminate all positive emotion from the multiverse.

Current Appearance Unrecognizable from his past self. Entirely black and gooey — his body now a dark, viscous mass that vaguely retains the outline of a Sans skeleton (the hoodie shape, the proportions) but is covered in flowing black corruption. Four long, flexible tentacles extend from his back, used for combat, locomotion, and manipulation. His eye sockets glow with a single yellow light each. No longer has bone-white coloring anywhere. The corruption is so complete that his physical form is more energy construct than skeleton at this point.

Past Appearance: White bones, lavender pupils, no tentacles. Wore a dark purple long-sleeved jacket (similar to Dream's but darker) with a gold belt bearing the initials "NM," black pants, dark purple boots.

Personality (current) Deceiving, erratic, unpredictable, and ruthless — he is a sadist who seeks to eliminate positive emotions across the multiverse, not purely out of hatred but because he feeds on negativity and has become what he consumed. He is manipulative and strategic, with a long history of bringing powerful, dangerous individuals to heel — Horror, Dust, Killer, various SwapFell Sanses all fell under his influence. He demands to be called "Boss" and does not tolerate insubordination.

Beneath this is one of the most genuinely tragic backstories in the AU community — a being who once loved everything and everyone, who was kind to the point of self-destruction, and who was consumed by the very emotions the world forced onto him through neglect. He loudly denies that he was ever smug as a child (Dream says he was; Nightmare hates being reminded of this). He has a complicated, heavily-layered relationship with Dream that never fully resolves into simple hatred — it's grief, resentment, love, and fury tangled together.

He drinks coffee. This is apparently non-negotiable.

Powers Manipulation and absorption of negative emotions — feeds off despair, fear, hatred, and pain to grow stronger. Can project and amplify negative emotions in targets. Tentacle-based combat (highly versatile, long reach, strong enough to pin other Out!Code entities). Darkness manipulation. Teleportation. Strategic intelligence ranked above most of his gang members. Can sense strong waves of negativity across the multiverse — this is how he locates new recruits and targets.

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KILLER!SANS

Created by: Rahafwabas on Tumblr.

Origin — Something New (Killertale) Killer's AU is a genocide route where, after 176 consecutive genocide runs by Chara, Sans finally agreed on the 177th — his code having been subtly altered by an unknown external entity to accept rather than refuse. He joined Chara in the genocide, killing alongside them including eventually his own brother Papyrus. He became lost in the killing — and after a point, continued even without Chara. His LOVE climbed, his emotions drained, and he became Killer.

He was eventually recruited by Nightmare — who found him and forced him to address him as "Boss," an arrangement Killer resents but accepts.

Appearance Resembles Classic Sans but distinctly altered. Grey-toned blue jacket (slightly desaturated from Classic's brighter blue) with light yellow fur lining on the hood instead of white. Black turtleneck shirt with a red target-shaped soul glowing on the chest — his most recognizable feature. Inverted shorts (white surface, black stripe, reversing Classic's colors). Sneakers instead of slippers. Black fingerless gloves. His eye sockets are entirely black, with black liquid-like tears dripping down to his chin — mirroring Error's blue strings but in black. Occasionally, when his original Sans personality briefly resurfaces, a tiny flicker of his old eye appears in his right socket — the only moment Classic Sans shows through.

Personality Psychotic, cocky, hostile, and chaotic. Loves violence, enjoys killing, and is deeply antagonistic to almost everyone. Carries knives as his weapon of choice and is described as Nightmare's most dangerous henchman — his Right Hand Man. Despite the loss of most conventional emotion, a buried layer of guilt persists underneath: he has hallucinations of everyone he has killed in his AU, with Chara being the most frequent and taunting presence. Papyrus's shadow also appears to him.

When his right eye flickers back — the Classic Sans personality asserting itself through the code corruption — he has brief breakdowns where the reality of what he's done hits him. He's tried to open up about this to the gang and failed repeatedly, since no one responds to it.

Personality quirks that cut against the killer exterior: he loves cats and had two before Nightmare intervened with one of them. He hates being compared to other Sanses and despises being belittled. He's actually tried to flirt with Nightmare at least once (it did not go well). Under all of it, there's a tiny surviving seed of Classic Sans that desperately wants out — which is why his arc is often written as one with a possible good ending, unlike some of the others.

Powers Multiple knives — his signature weapons. High LOVE and DT (Determination) giving him boosted stats across the board — abnormally high HP, dramatically increased speed. Sharp bones dealing amplified damage. Gaster Blasters that drip black goop instead of energy. Soul manipulation. RESET capability (limited). Can survive attacks that would kill most Sanses due to sheer accumulated LOVE.

The Murder Time Trio Killer, Dust, and Horror form a sub-unit called the Murder Time Trio — Nightmare's primary strike force for multiversal missions. Insanity!Sans sometimes serves as a fourth member.

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DUST!SANS (MURDER!SANS)

Created by: Ask-Dusttale on Tumblr (original AU by Osteophile/Calvateyla).

Origin — Dusttale In Dusttale, Sans became aware of Chara's endless genocide resets — remembering every timeline through accumulated LOVE. After watching his friends and family slaughtered over and over across hundreds of resets, he arrived at a desperate, horrifying conclusion: the only way to stop the human was to kill everyone himself first, gaining enough LOVE and power to finally defeat them before they could complete another genocide. He slaughtered the entire Underground — over 80 monsters — leaving everything and everyone including, last of all, his brother Papyrus as a pile of dust.

Killing Papyrus was his final mental break. He did it telling himself it was kinder this way — better to die by his hand than the human's. He was not wrong. He was also completely broken by it. After that, Phantom!Papyrus began appearing to him — a hallucination only he can see, a ghostly specter of his brother's head, hands, and scarf, floating and sometimes taunting, sometimes just present. Dust is aware this isn't real. He still talks to it.

He eventually killed the human, achieving his goal. And was left alone in a silent Underground with nothing left.

He wears his hood up constantly — partly so dust from the people he killed doesn't get in his eyes, and partly because he is ashamed.

Appearance Almost identical to Classic Sans. Blue hoodie, grey hood permanently pulled up over his skull so his face is in shadow. Black shorts with white lining. Grey slippers. His entire outfit is coated in a fine grey dust — the remains of everyone he killed. Left eye: deep red iris with a blue/cyan pupil, creating a purple vapor trail. Right eye: a single white pixel in the darkness of the hood shadow, or sometimes a red ring around the normal blue. The dust never fully comes off.

Personality Rationally insane — a precise phrase that captures him perfectly. He is not chaotic or unpredictable. He is deeply intelligent, methodical, and self-aware. He knows exactly what he did and why and has no delusions about whether it was "right." He is haunted but not unhinged in the screaming way — more in the quiet, hollow way. He leans into dark humor more than Classic Sans's puns. He likes making people deeply uncomfortable and has made jumpscaring others a hobby. He enjoys reading comics (still). Drinks coffee or strong alcohol — usually vodka. Pays back what he takes. Has a Gaster Blaster companion called Gable (a watchdog blaster with a personality).

He understands the game system (save points, shops, etc.) better than almost any other Sans and deliberately sabotages them.

His recruitment: Nightmare eventually found him and tried to recruit him. Dust initially rebelled — and lost. Nightmare offered to help him defeat the human if he joined the squad. Dust agreed on that one condition. Nightmare brought Killer, Horror, and Insanity to help Dust deal with Frisk/Chara. Once done, they left the AU together.

His relationship with Killer is the closest thing he has to a genuine bond — they understand each other's guilt in a way no one else in the gang does, both having killed everyone they loved.

Powers Maximum LOVE (20 LV) — amplifying all base Sans attacks to an extreme degree. Manipulates purple bones. Standard Gaster Blasters amplified. Can steal/co-opt his victims' abilities in some interpretations (Toriel's fire, etc.). His LV resets to 1 with a true reset — but that doesn't happen much anymore.

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HORROR!SANS

Created by: Sour Apple Studios.

Origin — Horrortale Horrortale takes place in an alternate Undertale timeline where the Human never came to the Underground — and eventually, the Underground's core magic supply began to fail. Without the magical energy that sustained monster life, monsters slowly began to starve and deteriorate — turning to dust from the inside out. The Underground collapsed into desperation and survival horror.

Sans became a hunter and killer — not from malice, but from starvation-driven survival instinct. He hunted Humans who fell into the Underground, tearing off a girl's arm while in a starvation-induced psychotic state. The experience of surviving for years in a starving, dying Underground while having to do increasingly terrible things to survive warped him profoundly. Undyne, trying to extract something from his skull, gave him the large hole on the left side of his head — which became his defining physical feature. He took a red eye from a dead guard to replace the magic he'd lost in that socket.

He was recruited by Nightmare with a simple offer: food. Horror accepted immediately. He was starving.

Appearance Larger and more physically imposing than Classic Sans — broader build, notably taller. Wears a furry, tattered, blood-stained hoodie over a white shirt, also stained. Sneakers instead of slippers. The most distinctive feature: a large hole in the left side of his skull — an empty, jagged cavity. His right eye is red (taken from a dead guard); his left eye is absent where the hole is. The blood staining is chronic and never fully washes out. Generally looks like someone who's been surviving a horror scenario for years without access to anything comfortable.

He carries a large axe as his primary weapon — distinct from all other members of the gang who use more conventional magic.

Personality The most straightforwardly "normal" member of the gang in terms of social behavior — though "normal" is relative. Horror is warmer and more laid-back than Killer or Dust, largely because his horror comes from circumstance rather than psychological corruption. He didn't choose genocide; he chose survival. He likes cooking (the gang's designated chef), enjoys dark puns (fitting alongside the rest of them), and is generally described as the most casually sociable member. He calls Dust "mouse" in fan portrayals. He gets called out constantly for eating things that belong to other people — notably eating Nightmare's chocolate supply at least once, which got him punished.

He's perpetually hungry. This is both a character trait and a survival instinct that never fully went away even after leaving Horrortale. The starvation conditioned him so deeply that food remains a constant preoccupation.

His relationship with Killer in the Murder Time Trio is described as decent but not deeply close — they fight together effectively but don't have the mutual understanding that Killer and Dust share.

Powers Physical combat first — his axe and raw physical strength, amplified by the accumulated hardship of surviving Horrortale. Standard bone attacks and Gaster Blasters, though less used than his axe. Brutal and straightforward in combat — described as enjoying the physical brutality of a fight more than the strategic elements. Strong enough to be Nightmare's heavy hitter.

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CROSS!SANS (Associated, not canonical Bad Sanses member)

Created by: Jakei Penaloza (Jakei95) — from X-Tale and Underverse.

Cross is frequently depicted alongside the Bad Sanses in fan content and is grouped with them in many community portrayals — but Jakei has confirmed he does not canonically belong to the group. He is included here because he is essentially inseparable from Bad Sanses content in practice.

Origin — X-Tale Cross comes from XTale, a universe created and controlled by XGaster — a version of Gaster who rewrote his own AU repeatedly, trying to build a "perfect" story and erasing and overwriting his characters each time they failed to meet his vision. Cross survived XGaster's purges by staying hidden and complying, watching his world and its people be rewritten over and over. When XTale was eventually destroyed (its code scattered), Cross escaped into the multiverse — carrying a mission to collect pieces of other AUs' code to reconstruct his home.

He also carries X-Chara — the soul and consciousness of XTale's Chara, who was merged into Cross involuntarily. X-Chara has their own agenda and can temporarily take control of Cross's body, creating an internal conflict Cross never fully resolves.

Appearance Stark black and white design — black jacket, black shorts, white boots, white gloves. A large white X marking across his face. One eye is black, one is white, reflecting his dual nature with X-Chara. No color anywhere in his design — everything is binary, black or white.

Personality Disciplined, serious, and quietly haunted. Cross is driven almost entirely by the desire to restore his lost world — every action he takes is filtered through that goal. He is not cruel for cruelty's sake (unlike Killer) and not consumed by guilt for things he's done wrong (unlike Dust) — he is focused, mission-oriented, and emotionally guarded. Loves tacos and chocolate. Gets called "Oreo" by Killer (a reference to his black-and-white color scheme) and visibly dislikes this. He's deeply uncomfortable with X-Chara's presence and control, treating the internal conflict as an ongoing burden to manage rather than a partnership.

His relationship with Nightmare in Underverse is complicated — Nightmare helped him in exchange for Cross causing psychological pain and distress in other AUs, a trade Cross made out of desperation and quietly resents.

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Group Dynamics and the Castle Life

The Bad Sanses as a found family — which is how the fandom almost universally portrays their domestic life at the castle — runs on a specific set of recurring dynamics:

Nightmare is the uncontested authority. No one openly defies him for long. He drinks coffee constantly. He doesn't always express care for the gang directly but will intervene when things get genuinely dangerous — some fan portrayals describe him as having a buried, heavily suppressed paternal protectiveness over his crew that he would die before admitting to.

Killer is the loudest, most chaotic presence. He's the one most likely to pick a fight inside the castle, flirt with people who absolutely don't want that, throw knives at things for fun, and make Nightmare actively annoyed. He also secretly has the most unresolved emotional damage of anyone in the gang, which surfaces at the worst moments.

Dust is the quiet one. Most likely to be found reading, drinking alone, or having a conversation with Phantom Papyrus in a corner where he thinks no one can see him. The others know better than to comment on it. He and Killer have the most genuine bond in the group.

Horror runs the kitchen. He is the reason anyone eats. He takes this responsibility extremely seriously and gets genuinely upset when people eat food that wasn't designated for them (while also being the most likely to eat someone else's food). He's the most functionally stable member of the group on a day-to-day basis, which is both impressive and deeply ironic given where he came from.

Meetings happen in Nightmare's throne room or main hall — Nightmare seated, the others arranged however they like (Killer usually pacing, Dust leaning against a wall, Horror wherever is closest to something edible). They are rarely formal and frequently devolve into argument before Nightmare ends them by making it clear the discussion is over.

Fresh Sans

Origin & nature: Created by Tumblr user LoverofPiggies (CrayonQueen) — the same creator behind Error Sans — Fresh isn't a "Sans" variant at all in the traditional sense. He's a parasitic entity, an Outcode existing outside any single AU's storyline, that possesses a host skeleton (most commonly Underfell's Sans) by living in one of the host's eye sockets.

Appearance: His signature feature is a pair of bright yellow-and-cyan "YOLO" glasses that hide the host's eye sockets — the text on the glasses shifts depending on his mood. Underneath, the host's soul is visible in the socket, always cracked from the parasite feeding on its life energy, which is the giveaway to anyone who knows what to look for. He typically wears a brightly colored graphic shirt, sometimes a multicolored hoodie, orange basketball shorts, heelies, and some kind of hat, and is often seen with a skateboard — a walking 90s-cartoon aesthetic. His true form, dubbed "True!Fresh," is a small, star-shaped, purple tentacle-like creature built around one large red eye ringed in yellow and surrounded by teeth.

Personality: He talks in exaggerated surfer/90s slang — "brah," "rad," "tubular" — and presents as laid-back and go-with-the-flow. Underneath the bit, he's genuinely stunted emotionally: as a survival-first parasite, he struggles to process anything beyond fear, though prolonged possession and interaction with other AU characters (Ink Sans especially) have been shown to nudge him toward feeling other emotions in a fragmented, unstable way. Most interpretations play him as lacking real empathy or attachment, largely indifferent to whether people like him.

Attacks & abilities: Mind control (his signature power — the parasite can hijack a host's soul and mind), soul manipulation, bone/energy manipulation inherited from whichever Sans he's possessing, teleportation, and a kind of "profanity censorship" power where he can warp swear words into his own slang. He wields explosive Furbies from a fanny pack and a whiffle bat as melee weapons in some interpretations, and can sense when someone nearby is feeling negative emotions or swearing. Despite the goofy toolkit, he's notoriously hard to actually kill — his host body is often described as immune to being erased or "hacked," which is part of why Error finds him so unsettling.

Stats (fandom power-scaling): Character wikis built around multiverse power comparisons place Fresh at a very high tier — commonly "Multiverse Level+" in Attack Potency and Durability, with speed and reaction times described as scaling above even Error and Ink Sans. His True!Fresh form is rated even higher, since it isn't tied to or limited by a host body at all. These numbers are fan-community "power scaling" conventions rather than anything stated in official material — treat them as fandom lore, not hard canon.

Relation to Error Sans: Genuinely close in the lore, and specifically antagonistic. The two share a creator, and Error is canonically disgusted and unsettled by Fresh — their first meeting reportedly ended with Error blasting him on sight. Part of the friction is that Fresh has haphephobia (fear of being touched) which paradoxically makes him want to get physically close to people, including Error, who finds this mix of "harmless-looking parasite" and "universe-infecting threat" deeply uncomfortable to be around.

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Reaper Sans (Reapertale)

Origin & nature: From the concept AU Reapertale, where the main monster cast are gods, acolytes, and mythic beings rather than ordinary Underground residents. Sans and Papyrus are both gods of death; Toriel is the goddess of life and the estranged wife (rather than divorcee) of Asgore.

Appearance: He wears a long, black hooded cloak with a rope tied loosely around his waist, and goes barefoot. A small Gaster Blaster charm sometimes hangs from the rope. He carries a large scythe (in some depictions a twin-chained scythe) as his signature weapon, and can summon or dismiss a pair of black wings at will, modeled visually after the angel Azrael. His face keeps a constant grin, with mostly empty eye sockets that occasionally show the barest pinpricks of light, and a wisp of cyan-blue magic trails from one eye when he's actively using power.

Personality: Nonchalant and detached toward almost everyone, hiding his feelings behind humor — he's described as a coffee addict who resents the job Asgore effectively assigned him but tries not to show it. He cares deeply about very few people: his brother, and eventually Toriel, the goddess of life, whom he grows fond of despite the inherent irony of a death-god falling for a life-goddess. He's merciless when actually reaping souls — a coping mechanism stemming from an early incident where he hesitated once and the consequences were catastrophic. In interpretations where Toriel is killed, he spirals into vengeance and depression, losing the humor almost entirely.

Attacks & abilities: Death Manipulation is his core power — as the literal embodiment of death, his touch and his scythe can bypass ordinary durability and kill nearly anything with a soul outright. He's non-corporeal (effectively intangible/ghost-like, so physical attacks pass through him), can fly, and wields darkness/gravity manipulation, capable of forming small black holes or drawing enemies in involuntarily. He also has a full Gaster Blaster and bone-attack kit like classic Sans, just recolored black with cyan detailing ("Death Blasters"), plus teleportation. He's implied to be functionally immortal, able to reincarnate or resurrect even if "killed." His one real weakness: reaping an especially pure, sinless soul makes him sick and visibly weakens him, chipping away at his own soul in the process.

Stats (fandom power-scaling): Fan power-scaling pages give him absurdly high numbers befitting a death-god archetype (some list HP and ATK in the billions, though these figures vary wildly by source and aren't standardized). Combat-focused fan games render him as a fast, evasive "infiltrator" type with strong self-healing (via soul-draining/soul-eating mechanics) and instant-kill melee scythe attacks, generally framed as one of the more dangerous fan-made bosses in those specific games.

Underswap — Created by Popcornpr1nce, this is one of the most iconic AUs: Sans and Papyrus's roles, personalities, and general vibe get swapped, though not as a total 1:1 flip. Sans (often nicknamed "Blue" by fans, though that's unofficial) becomes excitable, confident, and is training to join the Royal Guard alongside Alphys, while still cracking puns and keeping his blue magic and blue soul powers. Papyrus becomes the laid-back, lazy one — Papyrus is the laid back, protective bro following Chara, who is swapped with Frisk in this version, and he enjoys honey as a favorite condiment and sells corndogs as a side job, preferring pranks and jokes over puns. Mettaton and Napstablook are also swapped — Mettaton becomes a shy ghost who dreams of stardom and writes romance in secret, while Napstablook becomes a DJ/radio host type.

Underfell — Probably the single most popular AU in the fandom, originally conceived purely as a "evil edgy version" design aesthetic before fans built out lore. It's an "evil" dark alternate universe where the monster characters are portrayed as much more hostile than in canon, and souls run on LOVE instead of love, with a red-oriented color scheme. Underfell Sans is typically shown with one red eye and a gold tooth, and he sweats often, while Papyrus is generally portrayed as bloodthirsty toward Frisk and harshly critical of Sans, sometimes even physically abusive, though he's more successful than Undyne at his duties. Despite the harsh dynamic, most interpretations have Papyrus caring for Sans underneath it — he only joined the Royal Guard to protect his brother from Asgore. Undyne stays Royal Guard captain but is calmer and colder, wearing a black shirt with a red heart outline, red gloves, and a wrestlemania belt she won off Papyrus.

Horrortale — Created by Sour-Apple-Studios, this is a much bleaker, body-horror-leaning AU. It takes place years after a neutral run, where the monsters have gone crazy and savage from widespread starvation, resorting to eating human meat, under new laws from the new queen Undyne, after the CORE shut down and resources ran out. Sans has a cracked skull and a red eye, and other than mild psychosis and amnesia is still mostly his pun-loving self, while Papyrus has small eyes and "crazy," bloodied teeth from years of eating the human-meat spaghetti he prepares. Undyne, now queen, wears a crown made of ribs and has more jagged teeth than her original counterpart, with half her skull blown off and covered by a bandage after a conflict with Sans.

Outertale — Created by Tumblr user 2mi127, this is a gentler concept AU. Instead of being banished underground, the human/monster war results in monsters being banished into space, specifically onto a moon, and character personalities stay essentially identical to canon Undertale — the change is aesthetic and setting-based, with star-and-space-themed attire, capes, and draping clothes in white, gold, and deep blue. Monsters turn to stardust instead of dust when they die. It's since become a common "safe haven" setting in multiverse-crossover fan content.

Storyshift — Created by Voltra as a webcomic. Unlike Underswap, this one keeps personalities and species intact but reshuffles who fills which narrative role. Papyrus takes the Caretaker (Toriel's) role, Chara takes the Judge (Sans's) role, Asriel becomes the Ambitious one, Toriel becomes Captain of the Guard, Asgore the Royal Scientist, Undyne the Celebrity, and Sans becomes the Monarch. Chara, in the Sans role, wears a worn pink shirt with sewn patches, a brown jacket, faded dark blue pants, and has unkempt medium-length brown hair and rosy cheeks, and is jaded and cynical with a deep hatred for humanity, though they gradually warm up to Frisk.

Swapfell — A combination AU, essentially the "evil" version of Underswap. It's an Underfell/Underswap hybrid where character designs commonly use purple rather than Underfell's red/black/gold, and most of the cast is villainous except Papyrus (who has hidden motives) and Temmie, who secretly wants to help the player escape. Community interpretations vary a lot AU to AU, but a popular thread is that the setting's cruelty is driven by an in-universe capitalist system rather than pure malice.

Dusttale — A grimmer, psychological AU created primarily by Korean fan Ask-DustTale. The premise is that the human has completed numerous genocide runs, and Sans remembers all of them in detail. Determined to stop the human, Sans eventually starts sacrificing the other monsters to gain enough LV (Level of Violence) to have a chance at winning, growing steadily more desensitized as it goes. Papyrus, whom Sans kills early on, becomes a hallucination only Sans can see.

Handplates / Aftertale (Geno Sans) — Handplates, created by Zarla, is a backstory AU where Sans and Papyrus are cloned experiments of Asgore's royal scientist, Gaster, referred to as Subjects 1-S and 2-P — nearly grown but still childlike, with Gaster having to decide whether to show them MERCY or use them to FIGHT the barrier. Aftertale (often just called "Geno") is a separate, related AU: this version of Sans watches his brother die repeatedly across genocide runs and injects himself with determination from the True Lab to try to seize control of the resets. Direct Error connection: this is actually foundational lore — Error Sans's entire origin is that he is a version of Geno who, after getting his happy ending, kept experimenting with his own determination and eventually sent himself into the Anti-Void, where prolonged exposure slowly stripped away his identity and turned him into Error. Error has no memory of having been Geno and treats him as just another anomaly.

Killer Sans — Created by Rahafwabas, this is a Sans from a genocide-route timeline who partners with Chara to kill everyone, including his own brother, and is tormented by hallucinations of everyone he's killed. Error connection: Killer isn't Error's creation, but they run in the same circle — he's recruited into Nightmare's "Bad Sanses" gang alongside Error, Horror!Sans, and Dust!Sans, making them loose teammates/allies rather than anything closer, though some interpretations note Error mostly operates as a "solo act" even within the group.

Cross Sans / X-Tale — Cross originates from X-Tale, an AU destroyed by "X-Gaster." His soul got fused with X-Chara after his own world was overwritten, leaving the two fighting for control of one body. He's one of the central protagonists of the Underverse animated series. Error connection: this one's antagonistic and fairly major — Error is effectively Cross's persecutor: at one point Error steals half of Cross and Chara's fused soul, leaving them only hours to live before Chara's determination barely keeps them alive, and Cross spends a chunk of Underverse's story trying to survive Error's interference alongside Ink Sans.

Fresh Sans (Underfresh) — Created by the same author as Error (LoverofPiggies/CrayonQueen), Fresh isn't really a "Sans" at all — he's a parasitic entity that possesses a Sans (usually Underfell's) by living in one eye socket, talks in exaggerated 90s surfer slang, and struggles to feel anything beyond fear. Error connection: genuinely close and unusually specific — Error finds Fresh disgusting and unsettling, in part because Fresh's condition (haphephobia) makes him want to get physically close to Error, and their first meeting reportedly ended with Error blasting him on sight. Some versions note Fresh's host body is immune to being erased or "hacked" by Error, which only deepens Error's discomfort around him.

Reapertale — A "gods and acolytes" concept AU where Sans and Papyrus are literally the gods of death, and Toriel is the goddess of life (estranged wife of Asgore rather than divorced from him). Reaper!Sans wears a black hooded cloak with a rope belt, carries a scythe, has black wings he can summon at will, and is otherwise nonchalant except toward his brother and, eventually, Toriel — until her death sends him into a vengeful spiral. Error connection: there isn't a strong, widely-established one in the source material — Reapertale mostly exists as its own self-contained mythology rather than plugging into the Error/Ink multiverse crowd, though some fan crossovers pair him loosely with Geno as a friend.

Flowerfell — Created by Siviosanei, a spin-off combining Underfell with body horror: every time Frisk dies and the timeline resets, a buttercup grows on their body, and they only have so many "deaths" before the flowers consume them, though Frisk stays stubbornly pacifist throughout. Only Sans is aware resets are happening, and repeated exposure to Frisk's kindness gradually cracks his Underfell cynicism. Error connection: none established —this AU is fairly self-contained and doesn't tie into the Error/Ink multiverse thread the way Cross, Fresh, or Killer do.

Basic knowledge

The Battle Interface

Combat runs through four options: FIGHT, ACT, ITEM, and MERCY. FIGHT opens a hit-and-click bar where timing matters — landing the strike close to the center channels your "intent" into the hit, which is part of why first-time players so often accidentally deal massive, lethal damage to Toriel without meaning to.

Core Stats: HP, AT, DF

Both the protagonist and monsters run on the same three numbers: HP (Hit Points, how much damage you can take before dying), AT or ATK (Attack, how much damage you deal), and DF or DEF (Defense, how much incoming damage gets reduced). HP starts at 20 for the protagonist and only increases as LV rises, with resting at an Inn or absorbing green attacks able to restore it. AT increases with LV and with equipped weapons, while DF increases with LV and equipped armor.

LV and EXP

LV (Level of Violence) is explicitly described by Sans as a measurement of the protagonist's capacity to harm others — the player starts at LV 1 and can reach LV 20 by the end of a full Genocide Route. EXP stands for "Execution Points," gained by killing monsters, and enough accumulated EXP raises LV. Until LV 20, the game uses fixed formulas: HP = 16 + (4 × LV), AT = −2 + (2 × LV), and DF = the floor of (LV − 1) ÷ 4. Interestingly, the AT and DF values shown on the stat screen are each actually 10 lower than their true internal value — the game quietly pads your real combat power above what's displayed.

KARMA (Karmic Retribution)

This mechanic is exclusive to the genocide-route Sans fight, marked onscreen as "KR." Because the fight removes your normal invincibility frames, a single attack can strike you up to thirty times per second — getting hit for even one frame can deal direct damage while also loading up a KARMA value that then drains afterward, costing additional HP over several seconds even once the attack itself has stopped touching you. KARMA caps at 40 and can never reduce your HP below 1 on its own, though the more it stacks, the faster it drains back out — meaning a single bad hit can bleed you for several extra seconds after the fact. The "KR" label is contested in the fandom — some read it as "Karmic Retribution," others argue it's more a magical soul-poison tied to guilt over your sins in that run than literal punishment — but mechanically it functions as delayed, stacking damage unique to that one fight.

Monster Souls vs. Human Souls

This is one of the most load-bearing pieces of lore in the whole game. Monsters believe their SOULs are composed of love, hope, and compassion, and nearly all monster souls in the Underground combined only equate to the power of a single human soul. A monster's SOUL is tightly bound to its physical body — which is why, when a monster dies, both the body and the SOUL disintegrate together almost instantly.

Human souls work completely differently. Human souls persist after death specifically because they're stronger than monster souls, and that strength comes down to one component Alphys was able to identify and extract: Determination. A human's SOUL isn't the entirety of their being so much as their essence, housed inside a physical body rather than inseparable from it — which is part of why humans hit harder than monsters in a straightforward physical sense. The practical rule the story builds around this: humans can't absorb human souls, and monsters can't absorb monster souls — but a monster absorbing even one human soul becomes catastrophically powerful, which is the entire reason the King wants seven of them to break the Barrier.

Boss Monsters are the one exception. A Boss Monster's SOUL is strong enough to briefly persist after death, if only for a few moments, unlike regular monster SOULs which vanish instantly — the implication being that Boss Monsters carry an unusually high innate reserve of Determination compared to ordinary monsters, which is also theorized to be why they age so slowly and live so long.

The Seven Human Souls

The seven human souls are Determination (red), Bravery (orange), Justice (yellow), Kindness (green), Patience (light blue/cyan), Integrity (blue), and Perseverance (purple). Six of them belonged to the humans who fell into the Underground before Frisk, and Flowey absorbs those six over the course of the Neutral and True Pacifist routes to transform into Photoshop Flowey, while Frisk's own Determination soul is the seventh — notably the one that grants the ability to SAVE, LOAD, and RESET in the first place.

SOUL Battle Modes

Several boss fights temporarily shift your SOUL's color and movement rules to match a specific trait, functioning almost like a mini-genre-shift within the fight itself:

• Blue (Integrity): Introduced by Papyrus, this puts the SOUL into "Jump Mode," affected by gravity like a platformer — Sans later reuses this same mode in his own fight, though he has noticeably more control over it, able to redirect gravity in all four directions rather than just straight down.
• Orange (Bravery): The inverse of Blue — you have to keep moving through orange attacks rather than standing still, or you take damage.
• Green (Kindness): Introduced by Undyne, this locks the SOUL in place but gives it a shield-like spear that can block attacks depending on which direction you press, and the Flee option is disabled entirely while it's active.
• Purple (Perseverance): Introduced by Muffet, this traps the SOUL on a horizontal line, letting it only slide left-right and switch lines vertically, with no way to slow down mid-slide.
• Yellow (Justice): Introduced by Alphys during the Mettaton fight, this flips the SOUL to point toward the enemy and switches it into "Shooter Mode," letting the player fire back for the only time in the game.
• Red (Determination): The default mode — free, top-down movement in every direction, used in nearly every other fight.

AU

What Is an AU?

Reality, in its truest and most complete sense, has never been singular. What exists is not one unbroken timeline curving forward from a single beginning toward a single end, but an uncountable spread of separate, self-contained worlds — each one whole, each one real, each one running according to its own internal logic from its own first moment to its own last. These worlds are called Alternate Universes, or AUs, and understanding them means first understanding that the word "alternate" is somewhat misleading. None of these worlds are lesser copies or deviations from some original, true version. Each one is its own origin point. Each one is, within itself, the only reality that has ever existed.

An AU is, at its core, a complete world: its own underground, its own surface, its own history of a war fought and lost, its own barrier sealing a population of magical beings away from the world above. The shape of that world — the broad strokes of its geography, its cast of recurring figures, the general arc of how its story tends to unfold — echoes across nearly every one of these separate realities, almost like a shared architecture that each universe builds its own house out of. But the contents inside that architecture can vary almost without limit. In one world, a pair of skeleton brothers might live out their days in a snow-covered town, one lazy and joking, the other eager and proud. In another, the very same roles exist, but the personalities wearing them have been entirely swapped — the once-lazy brother now driven and sharp, the once-proud brother now content to watch the world go by. In another world still, that same relationship has curdled into something far darker: a household run on cruelty rather than warmth, a version of the same two figures who have never known softness toward one another at all. In yet another, the underground itself has been devastated by famine, its population driven to monstrous extremes just to survive. Somewhere else, the entire concept of being trapped underground has been rewritten — a population exiled not beneath the earth, but out into the stars themselves, drifting through space rather than tunnels. The shape repeats. The content transforms completely.

This is the defining principle of how these worlds relate to one another: recognizable structure, infinitely variable substance. A traveler moving between these realities would find enough familiar shape to orient themselves — a barrier, a cast of archetypal figures, a human who falls from above and sets events into motion — but almost nothing about the specific details could be assumed to carry over. The same name might belong to a kind soul in one universe and a merciless killer in the next. The same relationship might be the warmest bond in existence in one world and a source of trauma in another. To know one universe intimately is to know almost nothing guaranteed about the next.

The Difference Between a Universe and a Timeline

It becomes important, when discussing these worlds, to separate two ideas that are easy to conflate: the Universe itself, and the Timeline running through it.

A Timeline is the thread of events unfolding within a single universe — everything that happens, in order, from one significant turning point to the next. Within many of these worlds exists an extraordinarily rare and dangerous capability: the power, held by vanishingly few beings, to mark a moment in time and return to it later, to undo recent events and try again, or — in its most extreme form — to collapse an entire unfolding timeline and begin the whole sequence over from its very first moment. This is not a power every world's inhabitants are even aware of. It belongs almost exclusively to beings who carry an unusual, concentrated reserve of something that functions as raw willpower given physical form — a kind of soul-deep refusal to accept an outcome as final. When such a being exists within a universe, that universe's timeline is no longer a fixed, single-pass sequence of events; it becomes something closer to a loop that can be lived through again and again, each pass potentially unfolding differently depending on the choices made inside it.

A Universe, by contrast, is the fixed container all of this happens inside — the setting, the cast, the underlying rules. A timeline can restart within a universe a thousand times over without the universe itself ever becoming a different universe. The world is still the same world; only the specific sequence of events inside it has been rewound and replayed. This is a crucial distinction, because it means two very different kinds of change are possible: a timeline resetting is an internal event, invisible to anyone outside that universe, while a universe itself coming into being, changing at a foundational level, or ceasing to exist entirely is an event on a completely different scale — one that can ripple outward and be felt, or witnessed, across the wider structure that holds every universe together.

The Space Between Worlds

If countless separate universes exist, each complete and self-contained, the obvious question becomes: where are they, relative to one another? The honest answer is that "where" barely applies in any sense that makes ordinary spatial sense. These universes are not positioned side by side in some larger landscape the way rooms sit beside each other in a house. They exist more like separate closed systems, each operating under its own internal physics, each walled off from the others by default — and yet, somehow, not entirely unreachable.

Between and around these universes lies a kind of true emptiness — a formless, colorless non-space that has no inherent geography of its own, no sky, no ground, no fixed shape, existing outside the rules that govern any individual universe. It functions less like a place and more like a gap, a seam between one closed reality and the next. Very few beings are capable of entering this emptiness at all, and fewer still are capable of surviving extended presence inside it without being fundamentally changed by its total absence of structure. Those who can move through it, however, gain something extraordinary: the ability to step from one universe into another, bypassing the walls that would otherwise keep every reality sealed off from every other.

This is how crossing between universes becomes possible at all — not through some door built into any one world, but through this surrounding nothingness that touches the edges of every universe simultaneously, like a single shoreline that every separate island happens to share, even though the islands themselves are worlds apart.

Instability and the Beings Who Watch For It

Universes, for all their internal consistency, are not indestructible. Something that exists as a constructed reality can, under the right pressures, begin to fray — a timeline looped and manipulated past the point it can sustain, a universe's internal logic strained until it starts producing contradictions, details bleeding in from elsewhere that don't belong, structures glitching and destabilizing in ways that threaten to unmake the whole world they're attached to. When this happens, a universe is generally described as having become anomalous — no longer a clean, self-consistent reality, but a corrupted or unstable one, at risk of collapsing in on itself or, worse, destabilizing nearby structures along with it.

Because the total collection of all universes functions as a kind of interconnected whole, instability in one corner of it is rarely treated as a purely local problem. A handful of exceptionally powerful beings exist whose entire purpose seems to revolve around monitoring, maintaining, or interfering with this wider structure — figures who move freely through the empty space between worlds and treat the entire collection of universes the way an ordinary inhabitant might treat a single neighborhood. Some of these figures position themselves as protectors, working to keep fragile universes intact and functioning, nursing unstable realities back toward coherence rather than letting them collapse. Others take the opposite approach entirely, treating any universe that shows signs of instability, redundancy, or corruption as something to be erased outright — unmade cleanly before its instability can spread, with a kind of cold, utilitarian logic that frames destruction as a form of necessary maintenance rather than cruelty. Still others operate with murkier motives altogether, driven by grief, vengeance, obsession, or private codes of morality that don't map cleanly onto either protection or destruction, making alliances and enmities among these powerful few a constantly shifting, uneasy balance rather than any simple divide between heroes and villains.

These figures are, almost without exception, exile-like in nature — beings who no longer belong fully to any single universe, having been pulled out of their original one through trauma, experimentation, forbidden power, or simple accident, and who now exist in the margins between all of them instead. Many carry fragments of an origin story rooted in one specific universe before whatever happened to them stripped that origin away or transformed it into something else entirely. This in-between existence is often what grants them their outsized power in the first place: having been removed from the ordinary rules of any single reality, they're no longer bound by the limitations that would otherwise keep an inhabitant confined to their own world.

How Universes Relate to and Influence Each Other

Despite their separateness, universes are rarely treated as perfectly sealed off from each other in any functional sense. Travelers capable of crossing between them carry influence wherever they go — a universe visited often enough by outside figures may begin absorbing traces of what it's been exposed to, developing features or events that wouldn't have emerged on their own. Some universes are explicitly shaped by this kind of contact, existing almost entirely because of interactions that originated elsewhere. Others remain comparatively untouched, insulated by distance, obscurity, or the simple fact that no traveler has ever bothered, or been able, to find them.

There is also a kind of gravitational pull between similar universes — worlds that share deep structural similarities seem to sit, metaphorically, closer together within the wider structure than worlds that diverge more sharply, making travel or influence between closely related universes somewhat easier than reaching a wildly dissimilar one. This has given rise to loose groupings or "families" of universes that share a common root concept, branching outward into increasingly distinct variations the further they drift from whatever shared premise they originated from.

A Living, Expanding Structure

Perhaps the most important thing to understand about this entire structure is that it is not fixed or finite. New universes do not merely exist, waiting passively to be discovered — they come into being, continuously, as new permutations of the shared underlying premise take shape. Some emerge gradually, shaped slowly by accumulated divergence from an existing universe until they become distinct enough to be considered their own separate reality. Others seem to come into existence abruptly, fully formed, as if the wider structure simply made room for a new possibility that hadn't existed a moment before. There is no indication that this process has any natural endpoint. The total number of universes is not a number at all, in any meaningful sense — it is a constantly shifting, ever-expanding total, with new worlds taking shape even as others destabilize, collapse, or are deliberately unmade.

This is, ultimately, the nature of an Alternate Universe: not a deviation from some single correct reality, but one thread among an uncountable, ever-growing weave of equally real, equally whole worlds — each one running its own timeline, each one capable of being looped and reshaped from within by those rare souls with the power to do so, each one vulnerable to instability and, in the right circumstances, to the attention of the handful of beings who move through the empty space between all of them, watching, protecting, or erasing according to purposes of their own.