Author's Note:
This chapter will focus solely on the events unfolding within the Infinity Castle. The main character (MC) will not appear in this chapter.
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INFINITY CASTLE
THIRD PERSON POV
The Infinity Castle twisted like a fever dream, its corridors bending at impossible angles, walls pulsing with a sickly crimson glow. Stairs spiraled into nothingness, defying gravity, while faint echoes of distant screams bounced off the polished wood.
The air was thick and heavy with the metallic tang of blood and the faint hum of Muzan's malevolent will. Shadows flickered like living things, darting across the warped floors.
A hush fell—unnatural, suffocating, as if the castle itself held its breath. Then—
He emerged.
Kokushibo.
The silence shattered as his towering figure stepped from the shadows, his presence a cold blade slicing through the air. His six eyes gleamed under the dim light, serene yet burning with a quiet fury, like a storm trapped in a bottle.
His long, dark hair swayed slightly, framing a face that was both regal and monstrous. The grotesque katana in his hand—flesh and bone melded into a living weapon—pulsed faintly, as if thirsting for the fight to come.
His shoulders were squared, his posture unnervingly calm, but the slight tilt of his head betrayed a flicker of curiosity, a predator sizing up its prey.
Tanjiro Kamado stood rigid between Giyu Tomioka and Sanemi Shinazugawa, his heart hammering so loud he swore the demon could hear it.
His fingers tightened around his Nichirin Blade until his knuckles turned white, sweat trickling down his temple despite the icy air. His eyes, wide and earnest, locked onto Kokushibo, a mix of fear and defiance swirling in their depths.
"Upper Rank One…" he whispered, his voice steady but laced with a tremor only he could feel.
Giyu's face was a mask of focus, his dark navy blue eyes narrowing to slits as he sized up the demon. His jaw clenched, and his fingers flexed around his sword's hilt, betraying the tension coiled in his lean frame.
"Stay sharp," he murmured, his voice low and taut, like a drawn bowstring. His shoulders hunched slightly, ready to spring, but his calm exterior held—a still lake hiding a raging current beneath.
Sanemi, on the other hand, was all fire and fury. "Tch. Ugly bastard," he spat, rolling his shoulders with a loud crack that echoed in the eerie quiet. His scarred face twisted into a sneer, but his eyes burned with a reckless, almost manic glee.
He gripped his blade so tightly his knuckles popped, and the air around him seemed to shimmer with his barely contained rage. "This one's mine," he growled, his lips curling into a grin that was more snarl than smile, like a wolf baring its teeth.
Tanjiro's brows furrowed, his chest heaving as he stepped forward, his boots scuffing the warped floor. "No. Ours," he said firmly, his voice cutting through Sanemi's bravado.
His eyes flicked to his comrades, a silent plea for unity, his shoulders squared with resolve. "We won't survive if we fight him alone." His heart thundered, but his face softened for a moment, a flicker of warmth in his gaze—trust in his allies, faith in their shared strength.
Kokushibo offered no words, no taunts. His six eyes blinked slowly, unhurried as if the three Demon Slayers were merely an inconvenience. Then, in a blink, he vanished—a shadow swallowed by the castle's darkness.
CLANG!
The first strike hit like a thunderbolt, so fast it seemed the air itself screamed. Giyu reacted on instinct, his blade flashing up to meet Kokushibo's with a shower of sparks. The force drove him back, his boots carving twin trenches into the wooden floor.
His arms trembled, muscles straining, and a bead of sweat slid down his cheek, but his eyes never wavered—sharp, focused, unyielding.
"Water Breathing – Seventh Form: Drop Ripple Thrust Curve!"
He shouted, his voice steady despite the strain. His blade snapped forward, fluid as a river, curving toward Kokushibo's ribs with elegant precision. The demon twisted mid-air, his movements eerily graceful, his katana slicing downward in a deadly arc.
BOOM!
The impact shattered the floor like brittle glass, shards flying like shrapnel. Giyu staggered, his breath hitching, but his face remained a mask of grim determination, his eyes locked on his foe.
Sanemi didn't hesitate. "Wind Breathing – Third Form: Clean Storm Wind Tree!"
He roared, charging in like a hurricane unleashed. His blade spun in a whirlwind of slashes, each one howling with the force of his rage, aimed at Kokushibo's flank. His scars seemed to pulse with his fury, his teeth bared in a feral grin.
The demon's six eyes didn't flinch. With a single, effortless parry, Kokushibo deflected Sanemi's assault, his other hand still pressing Giyu back. The ease of it was infuriating, and Sanemi's grin faltered, replaced by a snarl as he dug his heels in, muscles bulging.
"You're slow," Kokushibo said, his voice cold as frost, cutting through the chaos. His lips barely moved, but the words carried a weight that made Tanjiro's skin prickle. The demon's posture was unchanged—calm, composed, a statue of deadly grace.
Then, a flicker of light. A rush of heat.
Tanjiro.
His heart pounded like a war drum, but his eyes burned with a fire that outshone his fear. His blade glowed red-hot, the Sun Breathing igniting every fiber of his being. His shoulders tensed, his stance wide, as he poured every ounce of his will into his next move.
"Hinokami Kagura – Flame Dance!"
He cried, his voice ringing with defiance. He spun forward, twin arcs of flame slicing through the air like falling stars, aimed at Kokushibo's back. His movements were fluid, almost desperate, his body trembling with the effort but driven by an unshakable resolve.
Kokushibo turned, his six eyes narrowing, a faint spark of interest breaking his icy facade. His head tilted slightly as if studying a curious insect.
"I thought only one remained who wielded the original form," he murmured, his voice low and thoughtful.
"So Muzan-sama was right to rage… You, boy—those Hanafuda earrings… You're the reminder of a ghost." His lips twitched, not quite a smile, but a flicker of something human—regret, perhaps, or recognition.
Their blades met in a blinding flash, crimson against shadow.
KRAKOOOM!
The shockwave tore through the corridor, splintering pillars and cracking the walls. Tanjiro was hurled backward, tumbling across the floor, his breath ragged as dust choked his lungs.
He hit the ground hard, his body screaming, but his eyes never left Kokushibo. His fingers tightened around his sword, trembling but unyielding, his face etched with stubborn determination.
Giyu intercepted another blow, his arms shaking as he parried, a thin line of blood trickling from his lip. His calm facade cracked for a moment, his brows furrowing in pain, but he held his ground, his stance steady as a rock in a storm.
Sanemi, ever the wild card, launched himself from above, his grin wide and unhinged. "Want wind? I'll give you a goddamn hurricane!" he bellowed, his voice dripping with reckless bravado.
"Wind Breathing – Fifth Form: Cold Mountain Wind!"
His blade spun like a buzzsaw, the air howling as he aimed for Kokushibo's throat. His scars gleamed with sweat, his eyes wild with the thrill of battle, but his movements were precise, every slash fueled by years of pent-up rage.
Kokushibo's lips curved into a slow, chilling smile—a ghost's grin, cold and haunting.
"Moon Breathing – Fifth Form: Rasetsu," he intoned
, his voice like a blade drawn across the stone. His katana unleashed a storm of crescent slashes, each one a shimmering arc of death that tore through the air. The room screamed as the crescents multiplied, slicing through space like a swarm of vipers.
Sanemi twisted mid-air, his reflexes were razor-sharp, but one crescent grazed his shoulder, spraying blood. He cursed, his grin faltering as he clutched the wound, his eyes blazing with defiance.
Giyu blocked two slashes, his blade a blur, but a third ripped through his haori, carving a red line across his ribs. He grunted, his face tightening in pain, but his stance remained firm, his eyes never leaving Kokushibo.
Tanjiro struggled to his feet, his body screaming in protest. His chest heaved, his breath ragged, but his eyes burned with an unyielding fire.
"I can't fall. Not here. Not now. Not while Nezuko's out there… waiting," he whispered, his voice barely audible but laced with steel.
The mark on his forehead flickered slightly, and his blade glowed a deeper crimson, like molten metal. His shoulders squared, his jaw set, and he gripped his sword with both hands, his knuckles white with determination.
Giyu wiped blood from his mouth, his movements slow but deliberate, one knee braced against the floor. His eyes flickered with a quiet resolve, his usual stoicism giving way to a spark of defiance.
Sanemi staggered upright, his grin returning, wild and unhinged, blood dripping from his shoulder.
"Heh… That all you got, you freaky bastard?" he taunted, his voice rough but dripping with bravado. He spat a glob of blood onto the floor, his scars twitching as he cracked his neck, ready for more.
Three warriors. Three breathing styles. One will.
They stepped forward in unison, their breaths syncing like a heartbeat. Tanjiro's fiery determination, Giyu's quiet resolve, Sanemi's reckless fury—they moved as one, a tide, a flame, a storm. "Hino," Tanjiro whispered. "Water," Giyu murmured. "Wind," Sanemi growled.
They charged.
Kokushibo's six eyes narrowed, his calm facade cracking for the first time. His grip tightened on his blade, both hands raising it high, his stance shifting to one of deadly focus. "So be it. Show me… your resolve," he said, his voice low and edged with something new—respect, perhaps, or anticipation.
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THIRD PERSON POV
The Infinity Castle's lower chambers churned with chaos, walls writhing like living flesh, their surfaces slick with a grotesque sheen. The air was thick with the stench of blood and scorched metal, the distant clanging of blades echoing like a warped lullaby.
Dim lanterns swayed overhead, casting flickering shadows that danced like specters. The floor pulsed faintly, as if the castle itself were alive, feeding on the violence within.
"Master, don't open your heart to that demon—he's already forsaken his humanity," Zenitsu shouted, his voice cracking with a mix of urgency and exasperation. His blonde hair was matted with sweat, his eyes wide and blazing with worry as he glanced at Jogoro.
His hands trembled around his blade, not from fear but from the sheer intensity of his emotions, his chest heaving as he fought alongside Cara against Upper Moon Five.
Jogoro's face was a storm of grief, his brows furrowed, his lips pressed into a thin line. His eyes, usually stern, shimmered with unshed tears as he stared at Kaigaku, his former disciple turned traitor.
His shoulders slumped slightly as if the weight of betrayal pressed down on him, but his grip on his sword remained steady, his knuckles pale.
"I thought my disciple might turn back," he said, his voice low and heavy, "especially with that demon cure, but it seems he's only sinking deeper into the abyss." His words dripped with sorrow, each one a wound he couldn't hide.
Zenitsu's fists clenched, his jaw tight, his eyes flashing with frustration.
"Ha-ha, Zenitsu, your master makes you look foolish," Kaigaku taunted, stepping forward with a cruel smirk. His dark hair fell into his eyes, but he didn't bother brushing it away, his posture cocky, almost lazy, as if the fight bored him.
His lips curled, revealing sharp fangs, and his eyes gleamed with malice, but a flicker of unease passed through them as he glanced at the chaos around him.
Cara, ever composed, stood tall, her eyes flicking to the grotesque figure at the center of the carnage. Her posture was relaxed but ready, her hand resting lightly on her weapon's hilt, her lips twitching into a sardonic smile.
"You disgusting lizard-looking poop," she said, her voice dripping with mockery as she eyed Gyokko. "You call that 'pot art'? Every artist out there might have a stroke hearing that." Her brows arched, and she tossed her hair back, her confidence unshaken despite the chaos.
Gyokko sat on his grotesque lotus throne, his serpentine body gleaming with porcelain scales, his multiple mouths and eyes shifting like a living nightmare.
His gills fluttered, and a high-pitched giggle escaped him, sharp and grating, like glass on bone. His head tilted, one eye narrowing as he sized up his foes, his clawed hands twitching with anticipation.
"You know nothing of art!" he hissed, his voice a venomous screech.
Zenitsu and Cara exchanged a quick nod. Zenitsu's trembling hands steadied, his eyes narrowing with resolve, while Cara's smirk widened, her fingers flexing around her weapon.
Without a word, she flung a pulse grenade at Gyokko's glaring eye, her movements swift and precise. The grenade exploded with a sharp crack, ultraviolet rays searing Gyokko's flesh, shrapnel tearing into his scales. His scream was guttural, his body writhing as smoke poured off him, his eyes wide with shock and pain.
Cara didn't pause. Her lightsaber hummed to life, its electric blue glow casting eerie shadows. "How do you like that?" she taunted,
swinging the blade with a flourish, the zuaah! zuaah! Sound cutting through the air like a bad sci-fi flick. She rolled her eyes, muttering under her breath,
"Alex and his stupid sound effects. If he were here, he'd probably just call me 'woman' and laugh." Her lips quirked, but her eyes stayed locked on Gyokko, her stance fluid and predatory.
Gyokko clapped his hands, and monstrous fish demons erupted from the walls, their bloated forms armored with molten teeth. The SPADE android, its body shifting like a living weapon, split into six plasma blades, slicing through the horde with mechanical precision.
Its steel wings flashed, cutting like razors, while its arms morphed into handguns, blasting through the demons with deadly accuracy. Zenitsu, still as stone, whispered,
"I'm not afraid anymore…" His eyes were closed, his face serene, but his body thrummed with energy, his fingers tightening around his blade.
"Thunder Breathing – First Form: Thunderclap and Flash!"
His voice is soft but resolute. He vanished in a burst of speed, the air crackling with electricity. Sixfold. Twelvefold. Twenty-fourfold.
A lightning storm tore through the chamber, yellow arcs zigzagging like divine wrath. Fish demons shrieked, then crumbled to ash, their forms disintegrating under Zenitsu's relentless assault.
The crack of thunder echoed, shaking the walls, and Zenitsu reappeared behind Gyokko, his blade inches from the demon's neck, electricity dancing across its edge. His eyes opened, blazing with quiet fury, his lips pressed into a determined line.
Gyokko's gills fluttered, his six eyes widening in panic. "W-WHAT?!" he stammered, his voice high and frantic, his body trembling as he realized the tide had turned.
The chamber erupted into chaos as Cara and the SPADE Unit swarmed in, UV grenades bursting like miniature suns, searing Gyokko's flesh. Cara moved like a storm in heels, her visor glowing as it tracked injuries. "Thirty casualties," she muttered,
slamming her lightsaber into a fish demon, her lips curling in disgust. "We hold this line." Her movements were precise, her body weaving through the fray with deadly grace, her sarcasm a shield against the horror.
Gyokko snarled, his pride crumbling under the assault. "You know nothing of art! I'll show you my grandest creation—a gallery made from your mangled flesh!" he screeched, slamming his palms to the ground.
"Blood Demon Art – Garden of Divine Flesh!"
The chamber twisted into a nightmare.
Walls pulsated like living flesh, ancient sculptures melted into grotesque, writhing abominations. The floor turned into a heaving sea of snapping jaws and squirming limbs—a demonic gallery of agony designed to crush minds and bodies alike.
A mutated, slimy arm lashed out of the floor, snaring Zenitsu by the leg, and yanking him toward the abyss of teeth and tendrils.
"ZENITSU!"
Cara's voice cut through the horror, raw with urgency and fury.
Without hesitation, she hurled a UV pulse grenade—a bright, spherical burst of concentrated light exploding midair.
BOOOOM!
The tendrils screeched as they vaporized in a burst of ultraviolet flame, releasing Zenitsu. He tumbled back, hitting the floor with a grunt, covered in viscous slime. He sat up, spitting and grimacing.
"Ugh… I'm gonna burn these clothes."
Cara was already moving—sprinting toward Gyokko, who leapt between his grotesque jars like a panicked rodent, his fish-like face twitching with fear. His body squirmed and flexed with unnatural elasticity, hideous limbs protruding where none should exist.
VWRRRRRRM!
Cara's lightsaber arced downward in a blinding slash, cutting through a mutated arm like butter. The air filled with the stench of scorched flesh and steaming ichor.
"Y-you… you lack art!" Gyokko shrieked, voice cracking as he ducked behind one of his ceramic monstrosities. "You're just a puppet!" As he already sensed there is no blood in Cara.
Cara stared at him, expression flat—unbothered. Her voice was calm, mechanical in tone but dripping with disdain.
"Do you even know how many lines of code are written into my neural chip, you amphibious joke?"
BOOM!
The sky itself cracked—as if lightning tore through the roof of reality.
A thunderbolt screamed downward, and with it—Zenitsu.
He surged upward into the air like a cannon, blood trailing from open wounds, his expression blazing with righteous fury.
His hair crackled. His aura roared.
"Thunder Breathing – Seventh Form: Honoikazuchi no Kami!"
Time slowed
His voice rang with unshakable resolve. A colossal dragon of lightning erupted from his blade, coiling in the air before diving toward Gyokko. The demon had no time to react.
Zenitsu's sword art engulfed him, blowing apart half his torso in a blinding flash. His screams echoed, lost in the thunderous roar of divine retribution.
{Thanks to Alex's glowing original healing potion, the Eite Demon Slayers fought with newfound strength, their bodies purged of toxins, their movements sharper, faster. The potion had transformed them, turning weeks of grueling training into raw, unbridled power. Every muscle, every breath burned with clarity. ~author note}
Kaigaku, locked in combat with Jogoro and the SPADE Unit, felt the weight of their enhanced prowess. Sweat poured down his face, his eyes darting nervously as he parried Jogoro's brutal strikes and dodged the unit's synchronized assaults.
His cocky smirk was gone, replaced by a grimace of fear, his shoulders hunched as he realized he was outmatched.
Upper Moon Five fell to Zenitsu's lightning, and Kaigaku's heart sank. Panic clawed at his chest, his breath shallow, his eyes flickering toward the shadows for an escape.
But before he could move—A pulse grenade came to his face and exploded which weakened his demon blood and cells.
then the opportunity arise for
"Thunder Breathing – First Form: Thunderclap and Flash: Divine Thunder!"
A golden streak of light passed through Kaigaku's body, so fast that even his advanced senses couldn't track it.
BOOOOM!
A thunderclap shattered the air. A golden streak of light tore through Kagaku, too fast for his senses to track. His eyes widened, his mouth frozen in a silent scream as his body followed his severed head.
Jogoro stood behind him, his katana sheathed, his face a mask of grief. His shoulders trembled, his eyes fixed on the ground, unable to meet Kaigaku's lifeless gaze.
"A true Thunder Breathing user must master the First Form before indulging in others… Didn't I teach you that, Kaigaku?" he whispered, his voice breaking with sorrow.
A pair of tears traced down his cheek, his hands clenching as he fought to keep his composure.
Kaigaku's head hit the ground with a dull thud, his eyes blazing with spite even in death.
"So what if I mastered it?! How could I ever defeat that monster—Upper-Rank two?! Even you would fall before him. And that pathetic loser, Zenitsu… he'd die like a DOG—!"
His words cut off as another head rolled beside his—Kokushibo's, its six eyes frozen in disbelief.
Kaigaku's arrogance shattered, his lifeless gaze locked on the fallen Upper Moon One, the monster he'd deemed invincible.
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