At twelve years old, Kuroda Ren hid beneath a collapsed pillar of a shrine in Hoshizora Ward. His eyes filled with fear, and he didn't know what was happening around him. Ren pressed his palms over his mouth to keep from making any sound. He only knows his father's words, said a few minutes back.
Ren's father hid Ren beneath the pillar, crouching low.
"Son, wait here. I'll check for your mother and sister," his father whispered.
Ren nodded and waited for his father.
Ren's father appeared after some time with Ren's younger sister, who tightly held their father's coat in her small fingers.
Then the world ruptured.
A tearing sound cut through the Ren's ears. Something sharp punched through his father's back and burst out the other side. His sister was thrown forward, her scream breaking into raw, she crawled back to Ren's father.
"Dad—Dad—!" she screamed.
Ren's body moved; he lunged from the shrine, gravel pierces into his knees, arms stretched—then he saw the eyes of something.
They glowed red in the smoke; anyone can say it was not human eyes.
The thing stepped forward in the smoke, dragging its claws free as if a knife cutting meat. Its shape was wrong, its limbs bent at angles, its skin stretched thin.
A demon.
Hoshizora Ward was almost dead. The demon lowered its gaze to his sister.
Ren froze.
His legs locked, every nerve screaming at him to run, to hide, to survive. The demon lunged at his sister.
Ren turned and crawled back beneath the shrine. He shoved himself into the darkness.
Through the cracks, he saw it.
He saw his sister stop moving. He saw his father collapse beside her. He saw red eyes lift, searching for other people.
By morning, Hoshizora Ward no longer existed.
Buildings collapsed, and no bodies were recovered. But a few people survived.
That was when the Demon Elimination Team arrived.
Steel-plated vehicles stopped at the destroyed area. Drones hovered, scanning for surviving people. Soldiers running with clinical efficiency, pulling survivors from debris, tagging the dead, recording data.
Ren was found half-conscious beneath the shrine. They wrapped him in a thermal blanket. Someone spoke gently.
Later—much later—they explained.
Ten years earlier, a distortion had opened a portal in the sky. From it came creatures, we don't know what to call, but because of their behaviour, everyone named them—demons. The first attack wiped out a city in minutes. Since then, encounters had multiplied. Humanity adapted the only way it ever had by weaponizing knowledge.
The Demon Elimination Army was born—advanced weapons, experimental technology, and soldiers trained to fight things that should not exist.
Ren listened.
They took him to Tsukimi Hill Orphanage, a quiet place far from the ruins. From that moment, he stopped crying. Grief calcified inside him, hardening into something sharper.
He began collecting information—incident reports, casualty numbers, grainy footage of demon encounters. Other children collected toys, but Ren collected data. He trained alone, long after lights-out, striking at shadows until his knuckles split.
By eighteen, he was recruited into the Shinkoku Demon Elimination Team.
On his first mission, his superiors noticed something immediately. Ren did not fight to neutralize. He fought to annihilate.
He tore through demons with methodical brutality, movements precise and merciless. Other demons hesitated when they saw him. Some fled, real fear flickered in their demon eyes.
At twenty-three, Ren became the top-ranked soldier in the team. But it wasn't enough.
The portals kept opening. Cities kept falling.
So beneath Kisaragi Research City, hidden under layers of concrete and clearance codes, scientists searched for another answer.
Dr. Amami Reiji stood at the center of it—mapping demon DNA, attempting the unthinkable: grafting it onto human biology to create soldiers who could match monsters' power.
Nine test subjects volunteered. Nine died violently.
One sample remained, but no one wanted to be next; no one wanted to die like that.
And Kuroda Ren sharpened his blade, unaware that the line between human and demon was about to collide—and that the revenge burning in his eyes would soon demand a terrible price.
One day, the news reached Ren. He gathered the data.
Ren stared at the data terminal long after the lights. Lines of text scrolled past his vision: nine had died, one sample remaining.
He closed the terminal and stood. There was no hesitation; he moved to the lab.
Within hours, he was on his way underground.
The elevators beneath Kisaragi descended far deeper. Layer after layer of security checks completed, armed personnel scanned his credentials. His name Kuroda Ren, was enough for the security members to remember. They didn't see Ren, but heard a lot about Ren.
The lab doors finally opened. At the far end waited the general—broad-shouldered, scarred, a man who had survived long enough to understand the cost of impossible decisions.
"You shouldn't be here," the general said without preamble.
Ren stopped three paces away and stood at attention. "I'm here for the sample."
Silence stretched between them.
"You've read the reports," the general continued. "You know the mortality rate."
"Yes," Ren replied.
"Nine out of nine," the general said sharply. "Dead, screaming violently. And you think you'll be different?"
Ren met his gaze. "I don't need to be different. I just need to survive long enough."
The general got angry.
"Ren, this project is above your clearance. Above mine. The top brass has already—" he said.
"I volunteer," Ren interrupted. His voice was flat and sharp.
"I won't be ordered. I'll sign whatever waiver you want," he continued.
"You don't understand what you're asking," the general tried to convince him.
"I understand exactly what I'm asking," Ren replied. "Power that scares demons. Power that closes the gap."
The general exhaled slowly, as if trying to bleed the anger out of his lungs.
"This isn't a weapon," he said. "It's an unknown. You could lose your mind, your body and mainly your humanity."
Ren did not look away. "I lost those things twelve years ago."
That ended the argument.
Dr. Amami Reiji was waiting inside the laboratory, surrounded by holographic displays and half-dismantled machinery. He looked older than Ren remembered—shoulders slightly hunched, dark circles carved deep beneath his eyes. He was a man who wanted success.
"No, absolutely not," Amami said immediately when he saw Ren.
Ren stepped forward. "You're the one who said the data wasn't complete."
"That doesn't mean I'd let a living soldier become my tenth corpse," Amami snapped. He gestured sharply, causing the displays to flicker. "The integration rate is unpredictable. Demon DNA doesn't behave like ours. It doesn't follow linear biology—it rewrites it."
"Then let it rewrite me," Ren said.
Amami stared at him; he saw the scars—old and new. He saw the stillness in the decision. Most of all, he saw the eyes. They were not the eyes of a man seeking power.
They were the eyes of a man seeking an end.
"Sometimes you have to hesitate," Amami said quietly.
Ren removed his jacket and placed it neatly on a nearby chair.
"Hesitation gets people killed," he said.
For a long moment, Amami said nothing. Then he turned away and began issuing commands. "Prepare the chamber. I want full monitoring. If his vitals spike beyond threshold, we abort immediately."
"And if they flatline as previously?" one of the technicians asked.
Amami's hands paused over the console. "Then we pray for him."
The chamber was circular, its walls lined with conduits that pulsed faintly with blue light. Mechanical arms unfolded from the ceiling like skeletal limbs. Ren stepped inside without assistance and stood at the center.
Restraints closed around his wrists, ankles, and torso to keep him from tearing himself apart.
Transparent tubes descended, connecting to ports embedded in the floor. The sample itself was contained in a reinforced capsule—dark, viscous, faintly luminous. Demon DNA did not resemble blood.
"Last chance," Amami said, his voice echoing through the chamber. "Once we start, there's no reversing the initial integration."
Ren lay back against the support frame. "Do it."
The tubes pierced his skin. The injection began slowly.
At first, it was cold. A spreading numbness that crept along his veins like frost. Then came the heat—sudden, violent, and unbearable. Ren's back arched; it was rewriting signals to his nervous system.
Pain detonated behind his eyes.
His heart rate spiked. Blood pressure climbed beyond safe limits. Neural activity flared erratically across the monitors.
"Stabilizers aren't holding," a technician shouted.
Ren screamed—it was layered with something deeper, it felt like an inhuman throat. His vision fractured. Memories surfaced and collapsed in rapid succession—his sister's face, his father's voice, red eyes in the smoke.
Then the monitors displayed—flatline.
The sound cut through the room like a blade.
"No—turn it off!" Amami shouted. "Shut it down now!"
Hands flew across controls. Power surged, then dropped. The chamber lights dimmed.
Ren's body went still.
In one terrible second, Ren died.
Then—
Beep.
A single line flickered.
Then another.
Ren's eyes snapped open.
The restraints groaned as his body convulsed—heart rate stabilizing, neural activity synchronizing, vital signs locking into place.
Amami staggered back, breath hitching.
"He's… he's adapting," he whispered. "He's actually adapting."
The tubes disengaged automatically, retracting into the ceiling. The restraints released. Ren sat up slowly, breath steady, eyes burning with a new, unfamiliar intensity.
The pain was still there—but it had changed. It felt like access.
Days later, when Ren could stand without assistance, Amami led him into a smaller chamber adjacent to the lab. Holographic equations rotated in the air, complex and unstable, folding in on themselves.
"Demon power isn't fuel," Amami explained shakily, as if afraid the words themselves might break. "It's a key."
Ren watched the equations warp space, light bending unnaturally around the projections.
"Demons don't move through space the way we do," Amami continued. "They cheat. They unlock shortcuts that shouldn't exist. And now—so can you."
He gestured to the center of the chamber. "If you focus your intent, you should be able to open a path."
Ren stepped forward alone. He closed his eyes.
He thought of Hoshizora Ward. Of red eyes in the dark. Of a world that needed fewer doors for monsters to walk through.
Something inside him responded. Space folded.
But instead of tearing open a portal, the chamber vanished.
Ren felt himself pulled away. Reality stretched thin, then snapped.
When sensation returned, he was standing on solid ground.
Darkness pressed in from every direction, vast and heavy. Ren was not alone. Something ancient shifted in the dark, aware of him.
Finally, I'm in the demon world, Ren thought to himself.