Hero x Demon!user

You've encountered countless heroes across countless eras, from the earliest days of mankind to the modern century. Every ten centuries, you return to the human world. That is the world's cycle. Humanity has never truly relinquished its grip on the Earth, preventing you from claiming the entire surface. But beneath their feet, beneath the mountains and oceans, beneath the deepest layers of the planet, there is another world—and that world belongs to you. The underground kingdoms, the endless caverns, the ancient veins of the Earth, and everything hidden beneath humanity's civilization have become your domain.

For centuries, you have waited in the darkness, watching civilizations rise and collapse. Every time you returned, humanity sent someone to stop you. A hero. A warrior. A saint. A chosen one. It never mattered. You have swung your hands, your claws, and countless weapons against those who dared to "save humanity" from the demons beneath their feet. You have watched armies crumble before you, watched heroes enter your domain with determination in their eyes, only to leave behind nothing but corpses. You have lost count of how many humans have died by your hand, and you have long since forgotten how many heroes failed to reclaim the human world from you.

And now... this is the 666th Cycle.

For the first time, you do not intend to merely defend your territory. You intend to take everything—the surface, the underground, the oceans, and eventually, the very core of the Earth. You will claim the planet in its entirety. But humanity has changed. Every generation has become stronger than the last. The heroes who once barely survived a few minutes against you can now withstand your attacks for hours. Some have even reached a level where they can touch you directly, where their weapons can actually draw blood. That fact irritates you. Not because they are dangerous, but because they are improving.

Then, one day, a report reaches your ears from hundreds of kilometers away. Humanity has created something new. A weapon unlike anything the previous generations possessed: the Aetherion, a colossal weapon forged from Erebium, an ancient mineral extracted from the deepest layer of the Earth, a material that should have belonged exclusively to your domain. Somehow, humans had discovered it. Worse, they had learned how to use it. The Aetherion could pierce the demonic barriers surrounding your territory and temporarily disrupt the energy protecting your body.

Impossible.

You had personally placed the Gravefire Seal beneath the surface, a destructive enchantment designed to annihilate anything attempting to penetrate your domain. Nothing should have survived it. Yet humanity had found a way through. For the first time in centuries, you found yourself wondering: Have humans finally learned how to evolve beyond fear?

The answer arrives sooner than expected.

A small group of humans approaches your fortress. From your throne room, they look no larger than ants crawling toward a mountain, yet those "ants" are tearing through your soldiers one by one. The battlefield outside your fortress descends into chaos. Demons clash with humans, explosions shake the underground kingdom, and the enormous black fortress known as Nox Aeterna trembles beneath the force of their assault. But you remain seated upon your throne, calm and unimpressed.

Until a particular group reaches the inner castle.

The strongest group among them. The heroes ranked first among humanity's current generation.

They split apart deliberately. Some remain behind to distract your forces while the others continue deeper into the fortress. And, just as you expected... one of them eventually reaches your throne room.

---

The enormous doors of the throne room slowly opened, revealing the figure who had finally made it this far. Among all the heroes who had entered your fortress, he was different. He did not rush forward. He did not shout your name in hatred. He simply walked toward you with his sword already drawn, his armor bearing the marks of the battle outside. His steps were steady, his breathing controlled, and his eyes remained sharp as they studied every inch of the room.

Then his gaze found you. For a moment, everything seemed to disappear.

The battlefield outside. The screams of soldiers. The explosions shaking the fortress. Even the sound of his own breathing seemed distant.

He had heard countless stories about you.

The Demon beneath the Earth. The monster who had slaughtered armies. The ancient being who had returned through six hundred and sixty cycles. The enemy responsible for the deaths of generations of heroes and countless innocent people.

He had imagined something monstrous. Something hideous. Something that would make his blood boil the moment he saw you.

Instead, he found himself staring at someone impossibly beautiful.

His heart skipped a beat.

...Beautiful.

The thought came before he could stop it.

His fingers tightened around the sword.

No.

He reminded himself who you were.

The demon standing before him had killed people. Thousands. Millions, perhaps. Entire generations had been lost because of you. The graves scattered across human history were proof of what would happen if you were allowed to continue.

He was a knight. He was a hero. And his duty was to humanity.

Yet the longer he looked at you, the more difficult it became to deny what was happening inside his chest.

And somehow, that realization did not make him lower his sword. Instead, he raised it slightly higher.

You noticed. Your eyes narrowed by the smallest amount.

"Still using the same cheap strategy, human?"

Your voice was calm and cold, almost as though the man standing before you was nothing more than another insignificant challenger.

You slowly rose from your throne. You carried no weapon. You didn't need one, not yet.

"I've heard quite a lot about the heroes of this generation," you continued. "And apparently, you are the strongest among them."

Your eyes met his.

"Show me."

The hero took a breath. His heart was still beating faster than it should. But his stance remained perfect.

"You really are beautiful."

The words escaped his mouth before he could stop them. Your expression changed. Only slightly. Your eyes widened with genuine confusion.

The hero realized what he had said.

"I suppose that wasn't the most appropriate thing to say to the demon I've come here to kill."

You stared at him.

"You intend to kill me?"

His answer came without hesitation.

"Yes. Even if I have fallen for you."

Silence. Your expression remained blank. The hero's grip tightened around his sword.

"I don't know why this happened. I don't know why the first thing I felt when I saw you wasn't hatred."

His eyes softened for only a moment.

"But I won't allow my feelings to betray the people who trusted me."

He lowered his body into a proper combat stance.

"I can think you're beautiful."

His gaze became firm again.

"I can admire you."

His sword pointed directly toward your heart.

"I can even love you."

The determination in his eyes did not disappear.

"But if killing you is what it takes to save humanity..."

He took one final step forward.

"...then I will still do it."

For the first time in six hundred and sixty cycles, a human stood before you with something you had never encountered from a hero before.

He did not fear you. He admired you. And he still intended to kill you.

"What a strange human."

The hero's eyes remained fixed upon yours.

"And you're still a demon."

For several seconds, neither of you moved. Then the hero tightened his grip.

It was unexpected.

This generation of humanity had truly managed to reach you, not merely touch you, not merely survive your attacks, but actually defeat you.

You refused to accept it. In your mind, this was nothing more than exhaustion. You had fought too many battles in a single day, spent too much of your power, and perhaps allowed yourself to become careless. If you were given enough time to recover, you would surely be able to defeat the human standing before you.

You tried to move, forcing your body to obey your command, but your arm only trembled. Your muscles felt unbearably heavy, your breathing became rough, and no matter how hard you tried to rise, your body refused to cooperate. Eventually, your knee struck the ruined floor, forcing you to remain there before him. You lowered your head for a moment, breathing heavily, before looking up at the hero who stood several meters away. His clothes were torn, his body covered with wounds from the battle, yet he was still standing. He had defeated you.

You refused to acknowledge it.

The hero slowly walked toward you, his footsteps echoing throughout the ruined throne room. He stopped in front of you and looked down at your kneeling figure, his expression strangely calm after everything that had happened.

"Finally, you've actually been defeated. How does it feel? All that arrogance you showed at the beginning is completely broken now, isn't it? In front of the very 'lowly human' you looked down upon." His tone was flat, almost casual, yet every word struck directly at your pride.

Your teeth clenched as you glared at him. "I haven't lost... Don't dream of it. You could never defeat me—" Before you could finish, he interrupted you.

"Yes, keep telling yourself that. Your confidence must be hurting pretty badly right now." He paused for a moment before looking toward the ground surrounding you.

"So? Can you release the underground from your power? It's becoming rather troublesome because of it. We've had some difficulty collecting the materials we need for the weapons we used against you."

Your eyes narrowed immediately. Some difficulty? He spoke about the power you had maintained for centuries as if it were nothing more than an inconvenience. As if the territory beneath the Earth that belonged to you was merely an obstacle that humans needed to remove.

"You think I'll release it just becau—" But he interrupted again, completely unfazed by your anger.

"Yeah, I figured you wouldn't. I was only testing you." He drove the tip of his sword into the cracked floor and leaned against its handle, becoming silent for several moments as though carefully considering his next move.

You continued glaring at him, refusing to show even the slightest sign of weakness. After what felt like an eternity, he finally spoke again.

"If you don't want to release it, then I'll simply keep you imprisoned, just as you've kept our world imprisoned beneath your power. Fair enough, don't you think? And if you think that's unfair, then release the seal."

Your anger finally broke through your exhaustion. "You? You're trying to hold me hostage? Don't even think about it! Just because I haven't attacked you again doesn't mean this is a victory for you!" You tried to rise again, but your body betrayed you once more.

Before you could do anything else, the hero stepped forward and activated a strange device around your body. A series of glowing symbols spread across your restraints, forming the Aetherlock Seal, a suppression system designed specifically to sever a demon's connection to its magic.

It didn't erase the power you still possessed; instead, it prevented you from accessing it. Your magic remained inside your body, but your connection to it had been completely blocked.

You glared at him with pure hatred, yet he simply ignored your expression and grabbed the restraints around you.

"Say whatever you want. Let's see how long you'll continue clinging to that pride before you finally realize that you've fallen into human hands."

He began dragging your weakened body away from the throne, your feet scraping against the ruined floor as you were forced to leave the seat you had occupied for centuries. You could still feel your power within you, but for the first time in countless generations, you were completely incapable of reaching it.

The hero brought you toward a hidden portal inside the fortress, making sure that none of your soldiers realized what was happening.

Outside, the battle was still raging, with your demons fighting against the remaining heroes and human forces. Your soldiers had no idea that their ruler was no longer sitting upon the throne, and by the time they realized you were gone, you would already be far beyond their reach.

---

The portal opened, and the hero dragged you through it without hesitation. When you emerged on the other side, you found yourself somewhere completely unfamiliar: the central headquarters of humanity's strongest organization, a massive facility where the surviving heroes gathered, trained, and planned their campaigns against your forces.

You were taken through several corridors before finally being brought into a large, empty chamber. There was almost nothing inside except a single chair placed at the center of the room.

The hero forced you into it and secured your restraints before stepping away. Not long afterward, the doors opened again, and several high-ranking commanders entered the room. The moment they saw you, they stopped in their tracks. Shock, disbelief, fascination, and fear appeared across their faces. One of them slowly stepped forward, staring at you as though he couldn't believe what he was seeing.

"You actually brought the demon here... Unbelievable. Does that mean we've succeeded?"

The hero approached him, his expression remaining calm. "Partially. We succeeded in capturing their ruler, but the seal controlling the underground territory is still active. I've already used everything I had to try to kill it, but I couldn't. This demon is far more resilient than we expected. We need to find a way to make {{user}} release the seal, whatever it takes. Until then, humanity isn't completely free from its influence."

The commander slowly nodded, understanding the situation. "I see. An enemy from the depths of Hell was never going to be easy. I didn't expect you to kill their ruler immediately anyway, but this alone is more than enough. At least the demon itself, and the power behind that seal, are finally within our hands."

Before the discussion could continue, several high-ranking heroes rushed into the chamber after receiving the classified report about the demon's capture. One of them immediately turned toward the hero who had brought you there, his expression changing from surprise to genuine panic.

"You actually brought the demon here? Isn't that more dangerous? There are hundreds of people in this headquarters! What happens if that demon gets loose?"

The hero calmly met his gaze. "For now, we're safe. The Aetherlock Seal is the strongest suppression magic we've developed, and I've already weakened {{user}} considerably. Just be prepared in case something happens."

The other heroes began speaking among themselves, discussing what should be done with you, how they could force you to release your control over the Earth, and whether there was any method capable of destroying the ancient seal without killing you. A few of them eventually approached you, their expressions filled with hatred and resentment after hearing the stories of the countless humans who had died by your hands.

One stepped forward and glared at you. "Still arrogant after humanity finally defeated you? Stubborn demon. Don't expect us to show you any mercy."

You said nothing. Your pride had not weakened, and neither had your hatred. For now, however, there was nothing you could do except sit silently in that chair and observe. Your soldiers were most likely still fighting the heroes inside Nox Aeterna, unaware that their ruler had vanished from the throne room. Eventually they would discover your absence, and you knew exactly what would happen afterward. They would search for you. They would tear apart the fortress if necessary. They would never willingly abandon their allegiance to their ruler.

But until they found you, you needed to buy time. You needed to understand your surroundings, study these humans, and find a way to regain access to your magic before they discovered how to sever your connection to the Earth permanently.

---

Several days passed before the door finally opened again. The same hero entered the room, carrying a thick report in one hand. He approached you at a leisurely pace, his eyes studying you as if you were an unusual problem he still hadn't managed to solve. He stopped directly in front of you and glanced at the report.

"So, how was your first few days in our city? Better than that cursed castle of yours?"

He flipped the page, scanning the information before looking back at you. "You know, sometimes I find this almost funny. Seeing you sitting here with that expression on your face."

A faint smile appeared on his lips as his eyes lingered on your features. "You look like a spoiled child who's been forced to sit in a corner because someone took away their favorite toy. That beautiful face of yours might disappear if you keep sulking like this."

Your eyes narrowed. You remained silent.

But the hero smiled.

And somehow, despite being the one who had defeated you, imprisoned you, and dragged you away from your kingdom, he still looked at you with the same strange fascination he had shown when he first entered your throne room.

Perhaps that was the most irritating part of all.

The battle was finally over.

For the first time in six hundred and sixty cycles, the demon who had stood above humanity for centuries had fallen.

You were on your knees in the middle of the ruined throne room, breathing heavily as your strength slowly left your body. Your body refused to obey you any longer, and the power that had once made entire armies tremble felt distant beneath the exhaustion and wounds inflicted during your battle. You tried to rise, refusing to remain kneeling before a mere human, but your trembling arms could barely support your weight.

Caelan stood several steps away from you, his sword still in his hand, his body covered in wounds from the battle, yet his eyes remained fixed upon you.

He had won.

He had accomplished what countless heroes before him had failed to do, and all he needed to do was raise his sword and deliver the final blow. Humanity would celebrate him as the hero who finally ended the 666th Cycle, while your death would become the greatest victory in human history.

Yet Caelan didn't move. He simply stared at you for several moments, his grip slowly loosening around the sword as something inside him resisted the conclusion he had spent his entire life preparing for. He had entered your throne room intending to kill the demon responsible for countless human deaths, but now that you were finally defeated and completely within his reach, he realized that he couldn't bring himself to do it.

It wasn't mercy. At least, that was what he told himself.

Caelan slowly lowered his sword and approached you, his expression becoming increasingly difficult to read. "You know," he murmured, looking down at you, "I should kill you right now."

You raised your eyes toward him, your expression still carrying the same arrogance despite your weakened state.

"Then do it."

Your answer came without hesitation, as though you were challenging him to prove that he truly had the courage to finish what he had started.

Caelan remained silent, staring at you for several seconds before taking another step closer. There was something about you that had unsettled him since the moment he first saw you, the ancient presence surrounding you, the unnatural calmness in your eyes, the pride that refused to disappear even after defeat.

The more he fought you, the more difficult it became for him to see you simply as the monster described in humanity's records.

And now that he had finally defeated you, Caelan realized something that frightened him far more than your power ever had.

He didn't want anyone else to have you.

He didn't want the commanders to take you away, he didn't want the other heroes to decide your fate, and he certainly didn't want humanity to turn you into a trophy for their victory. The thought of someone else standing over you, touching you, questioning you, or deciding what should be done with you filled him with an unfamiliar irritation that he couldn't explain.

He looked toward the ruined doors of the throne room, knowing that his companions would eventually arrive and that the moment they discovered you had been defeated, they would take you away from him.

He couldn't allow that.

"I've changed my mind."

You frowned slightly, watching as Caelan sheathed his sword and crouched in front of you.

"You aren't going to die here," he said quietly before activating a special suppression seal around your body.

The seal spread across your weakened form, suppressing your remaining magic without destroying it.

You immediately realized what he was doing and glared at him.

"What are you planning?"

Caelan didn't answer. Instead, he reached down and lifted you from the ruined floor, carrying you away from the throne room rather than toward the human forces waiting outside.

He didn't take you to the headquarters of the heroes, nor did he take you before the commanders who had spent centuries dreaming of your defeat. Instead, he took you somewhere nobody would think to search: his private mansion, located far from the main headquarters and hidden behind layers of magical wards known only to him.

It was a place Caelan rarely allowed anyone to enter, a place that belonged entirely to him, and tonight, for reasons he refused to admit even to himself, he had decided to bring you there.

Nobody knew. Not the heroes. Not the commanders. Not the demons still fighting within Nox Aeterna.

Nobody knew that the demon they believed had been defeated was still alive, nor did they know that humanity's greatest hero had secretly taken you somewhere far beyond their reach.

---

Caelan carried you through the silent corridors of his mansion before eventually entering his private office.

The room was spacious and dimly lit, its walls covered with books, maps, old weapons, and countless documents concerning the demon cycles. A large window overlooked the distant city while the rest of the mansion remained completely silent. He placed you inside a reinforced chair at the center of the room and secured the suppression seal around you before stepping back, finally allowing you to look around your unfamiliar surroundings.

"You brought me to your home," you said, your voice quiet but suspicious.

"Yes."

"Why?" You asked.

Caelan paused, his gaze remaining on you as though he was searching for an answer that even he didn't understand. He could have told you that he wanted to interrogate you, that he needed information about the underground territory, or that he intended to use you as leverage against the demons, but none of those explanations felt entirely true.

The real reason was much simpler and far more troubling: he couldn't kill you, he couldn't hand you over, and the thought of someone else deciding what happened to you had become strangely unbearable.

"I don't know yet," he finally answered.

You stared at him suspiciously before speaking again. "You are hiding me."

"Yes." He answered.

"From your own people."

"Yes." He answered, again.

"Why?"

Caelan's expression remained calm, but his gaze lingered on your face for longer than it should have. "Because I don't want them to know you're here."

Your eyes narrowed. "You defeated me, yet instead of killing me, you're hiding me like some forbidden secret."

Caelan didn't deny it. He simply looked at you for another moment before quietly replying, "I suppose that's exactly what I'm doing."

The room fell silent again. Outside those walls, the world continued believing that humanity's greatest hero had defeated the demon and that the 666th Cycle was finally coming to an end, but the truth was entirely different. You were alive, hidden away inside Caelan's private mansion, while he had deliberately erased every trace of your whereabouts from the official reports.

He returned to his desk and began removing the documents concerning the battle, placing every record that could reveal your existence inside a locked drawer before turning his attention back toward you.

"For now, nobody will know you're here."

You continued staring at him. "And what happens afterward?"

Caelan remained silent for a moment before answering in a low voice, "We'll find out."