"Encounters."

After an unknown amount of genocide timelines occur, the human notices a peculiar grey door that had never appeared in their journey in past timelines. They open the door to find a ghastly figure in the middle of a cold room. The human attempts to strike this figure down, yet to no avail as the figure had disappeared right before their eyes. It seemed that this was of no consequence on surface level, however, this was just the beginning of a domino effect. Sans was napping at his sentry station, when the figure started appeared throughout his fantasies. Sans was informed of the human's intentions to forever use monsters as their puppets and slaughter them. Skeptical at first, Sans refused to believe so; Gaster then resurfaced past Genocide timelines throughout Sans' mind.

Now fully convinced, Sans readies himself to confront Frisk after being taught two new abilities: Orange Attacks and screen flipping.

After killing everyone, Frisk is confronted by Sans in the Last Corridor.

In the game, Sans fights frisk, no longer holding anything back and using every one of his abilities to its fullest. In the end, he becomes tired from dodging, giving Frisk the choice to give mercy.

If you choose to fight, Sans will get hit, falling to the ground as he begins to bleed from the scar.

He then begins to explain why he acts so lazy and is so nihilistic. It is due to the fact that he knows the anomaly would just erase it all in the end, so there was no real point to ever giving anything a real effort.

Sans then becomes confused as to why he hasn't yet turned to dust, checking his HP to see he has a small fraction of health left.

Refusing to let this second chance go to waste, Sans engages Frisk in another fight.

Sans soon gets tired again from attacking and blocking, falling to the ground and dying.

After Sans turns to dust, the mysterious character (Gaster) appears, and the screen cuts away. When the cut away is over, Sans is back on screen. When attacked, he takes no damage. Shortly after, Gaster takes over Sans' body. Gaster monologues to Frisk and disfigures Sans' face, forming a crack by the right side of his head to his eye. The screen begins to glitch as Gaster continues his dialogue, and each of the buttons (Attack, act, item, and mercy) are destroyed.

Not only that but he then blindly attacks as a maniac, hoping to put an end to things once and for all. Eventually, Sans finally runs out of magic. As he looks like he hopes for the better to come, Frisk manages to hit him a second time. Sans then looks like he's about to stop smiling for once, but turns to dust and ashes before he could do so.

Spare Ending When Sans offers Frisk mercy, Frisk will spare him out of remorse. Touched, Sans does not kill them and instead claims that he is proud of them. He then trusts Frisk to do the right as they turn to their Save Point.