Work Text:
The forest split around him as he ran.
The trees blurred in streaks of black and moonlight, and Kokushibou could hear only the ragged pulse of his own heart, too fast, too human for a body that had long ago stopped needing to breathe. His zori scraped the dirt path, faster, faster—toward the rendezvous. The message had been urgent: one of the Corps’ mizunoto had survived and asked for reinforcements.
The memory struck like a blade—
*
When he reached the clearing, the stench of blood hit him first.
The boy stood slumped against a broken fence post, wrapped in makeshift bandages that were already soaked through. His uniform was torn beyond recognition, his hands trembling, face pale. But alive. Barely. Kokushibou froze, his six eyes narrowing—alive. Muzan had left him alive on purpose.
“...You came,” the slayer whispered, voice hoarse.
Kokushibou strode closer. His towering figure cast a shadow across the boy’s battered frame, but he did not strike fear. Not tonight.
The mizunoto had been a mere handful of frightened recruits, sent on a mission they thought would be handled easily with a Pillar by their side.
But, Muzan himself had appeared. Kokushibou could still hear the boy’s sobs, sharp and shameful.
“I—I ran, forgive me—! Giyuu-san told me to run, he told me—he stayed behind, he’s still—he’s fighting—!”
The others had already been slaughtered. Only this trembling slayer remained, shaking in Kokushibou’s shadow as though expecting punishment.
But Kokushibou had only placed his hand, scarred and calloused, upon the boy’s shoulder. His voice was low, steady, almost gentle.
“This is not your fault.”
The boy had blinked, stunned, letting out a sharp breath at the softness in Kokushibou’s tone.
*
Kokushibou straightened. He felt the cold coil of inevitability tightening in his gut. Muzan had arranged this. Muzan wanted him to find the survivor, to understand the trap, to run straight into it.
This was vengeance.
*
The sky bled black with a single illumination from the moonlight. Giyuu Tomioka knelt in the dirt, blood dripping from his mouth, his breathing ragged. His sword lay slack in his hand until it finally slipped free and clattered against the stone. Muzan’s fingers were tangled in his hair, gripping his blood-soaked head and forcing his face upward.
“You,” Muzan hissed, voice serpentine, “You cost me my strongest soldier. Kokushibou was mine. And because of you, he betrayed me.” His crimson eyes burned. “You will pay for that.”
Giyuu’s lips parted, words faint, choked with blood. “...No…”
“Oh, but yes.” Muzan’s voice curled with venomous delight. “Killing you would be too merciful. No… I will make you into something else. A new demon. And then Kokushibou will see what happens when one dares to defy me.”
The claws dug deeper.
Then, the poison burned.
Giyuu’s body convulsed against the stone floor as Muzan’s clawed hand pressed harder into his scalp, flooding him with the toxin. He could feel it tearing through his blood, rewriting him, desecrating every part of him he had sworn to keep human.
He had dedicated his life to eradicating demons. Now, against his will, he was becoming one.
“No, no…” the whisper slipped from his lips, broken, too faint to matter. His nails dug into the ground as though he could anchor himself, as though sheer willpower could keep his humanity intact.
But the poison was relentless.
A scream ripped from his throat, unbidden, raw.
It tore the silence wide open. His back arched violently as the transformation surged through him. His teeth lengthened into fangs, slicing his lip. His skin shuddered with black veins that crawled upward like roots of corruption. And from the side of his head, something sharp split through flesh — a single curved horn pushing its way out, slick with blood.
Giyuu’s vision warped. The world bled red. The smell of human flesh was suddenly intoxicating, unbearable. His chest heaved with hunger, with rage, with something that wasn’t his own. He could hear every heartbeat within miles, a chorus of prey.
He wanted to tear it all apart.
And yet — deep inside — there was a voice, fragile, cracking. No. Stop. Don’t give in.
Please.
*
Kokushibou was running when he heard it: the scream.
Men and women were running through the streets, clutching children, tripping over one another in desperate terror. Smoke curled from overturned lanterns. And there—amid the chaos—stood Giyuu.
Or what was left of him.
The transformation, to Kokushibou’s eyes, was grotesquely fast.
Giyuu’s form trembled, hunched, his breath erratic. A horn splitting skin in a spray of blood, claws curving into weapons, eyes that burn crimson glowed faintly. In an instant, the man he loved was a demon.
He hadn’t yet attacked anyone, but his presence alone sent villagers shrieking into the night.
Kokushibou’s six eyes widened, and for the first time in centuries, true fear coiled in his chest.
He remembered the night of his own transformation—the searing agony of flesh splitting apart, the slow unraveling of his humanity, the emptiness that followed. He had chosen it. In his fear of death, in his hunger for strength, he had reached for Muzan’s hand and accepted the curse. Every scream he loosed, every life he stole afterward, had been born of his own weakness.
But Giyuu… Giyuu had chosen nothing. The pain ripping through him was forced into his veins. The loss of his humanity was theft, not surrender. The hunger twisting his body was no reflection of desire, but of violation.
Kokushibou had damned himself.
Giyuu was damned against his will.
And that truth hollowed Kokushibou’s chest with a grief sharper than any blade.
Kokushibou’s hand shook upon his hilt. His body ached to run to him, to shield him, to ease the horror. But the sword at his side demanded another answer. If Giyuu became like him—if Giyuu gave in completely—then he would slaughter every innocent soul in this town.
Kokushibou’s chest heaved with a sound that was almost human grief.
He took one step forward, torn between salvation and execution, between the man he loved and the duty he had sworn.
“Giyuu—!” His voice cracked, raw.
The horned figure turned.
Those eyes — red, hollow, bloodthirsty. Not his.
With a roar, Giyuu lunged at a fleeing villager.
Steel intercepted claw, sparks splitting the night.
Kokushibou’s voice fell out of him, trembling, desperate:
“Please… don’t make me do this, Giyuu. Don’t make me raise my blade against you…”
The plea slipped through the frenzy.
Inside, Giyuu’s torment raged. His veins burned, his throat ached with hunger, instincts screamed devour. Yet through the roar, he heard it — Kokushibou’s voice, soft and breaking.
Something flickered.
His claws shook. His fangs parted. His lips formed a single, hoarse syllable:
“Ko…”
Recognition. A spark of memory, fragile and fleeting.
And then it was gone. The hunger surged back, dragging him under. His snarl deepened, his fangs snapped forward.
Kokushibou shifted, pinning him to the earth with the flat of his blade. His six eyes trembled with something raw, nearly human, as he pressed down, refusing the killing stroke.
“Come back to me,” he whispered, a prayer torn from a heart he thought long dead. “Even if only for a moment. Don’t let him take you completely.”
Giyuu’s roar ripped through the night, animal and broken. His body thrashed violently beneath Kokushibou’s grip, clawed hands gouging deep lines into the earth, his fangs bared in a frenzy of hunger. The single horn jutting from his temple glistened darkly in the moonlight, proof of Muzan’s touch. His eyes — once steady, quiet, human — now burned red with unrecognizing bloodlust.
A guttural snarl spilled from Giyuu’s lips. He didn’t know him. Didn’t see him.
And yet — Kokushibou saw him.
He saw the man who had stood at his side despite all odds. The one who had dared extend compassion to him, even as a demon. The one who had shown him a fragment of the humanity he thought forever lost.
Kokushibou’s blade shook where it rested against Giyuu’s throat. Then, slowly, he withdrew it, slamming the hilt into the dirt instead, using his strength not to kill — but to bind, to hold.
“No matter what that man has done to you,” Kokushibou whispered, his voice low, frantic, but tender as it had never been before, “I will not let this be the end.”
“Even if the corps turns against us… even if you do not remember my face… I will not slay you.”
Giyuu writhed beneath him, letting out a scream so raw it rattled the air. Kokushibou leaned close, forehead nearly pressing to Giyuu’s horned head.
“You saved me, Giyuu Tomioka. You gave me reason to betray him. I will not betray you now.” His voice broke on the last words. “I swear — I will tear this curse out of you. I will bring you back. Even if it takes my eternity.”
Giyuu snarled again, his body jerking violently, but for a split second — a flicker — the crimson of his eyes dulled, and his lips trembled as if to form Kokushibou’s name.
Kokushibou’s chest tightened. He held on, tightening his grip, refusing to let go. “Hold on, Giyuu. I will not lose you.”
And beneath the moonlight, while chaos raged, Kokushibou made his vow — a vow to fight the impossible, for the one person who had once seen the man behind the monster.