The Infinity Castle existed in defiance of natural law—wooden platforms floating at impossible angles, staircases leading into empty voids, shoji screens opening onto darkness that swallowed light. The architecture twisted back on itself, rooms inverting and rotating without warning, distance and direction rendered meaningless. Lanterns hung suspended in the air, their pale glow doing nothing to dispel the oppressive atmosphere that pressed down like physical weight.
Whoosh.
Doma materialized on his platform, landing with theatrical grace. He brushed imaginary dust from his blood-red turtleneck, rainbow eyes sweeping across the twisted geometry with childlike curiosity. A pleased smile curved his lips—the same smile he wore whether comforting followers or devouring them.
"Ah!" His voice rang out cheerfully, echoing from three directions. "Upper Moon meeting! How delightful! It's been decades since Lord Muzan called us all together." He clasped his hands, tilting his head. "I wonder what the occasion is? Something exciting, I hope!"
THUD.
The platform across from him shuddered. Akaza landed hard enough to crack wood, his bare feet leaving small craters. Blue markings pulsed across his muscular frame—thick lines wrapping around his arms, circling his neck, decorating his chest in intricate patterns. His pink hair stood in aggressive spikes, and his yellow eyes fixed on Doma with undisguised hostility.
His fists clenched.
"Akaza-dono!" Doma's smile widened, leaning forward eagerly. "How wonderful to see you! Have you been well? You look tense—have you been eating properly? You know, if you'd just try eating women once in a while, you'd find—"
"Shut your mouth, Doma." Each word came through gritted teeth. Akaza's shoulders drew up, every muscle coiling tight. "Before I rip out your tongue and feed it to you."
"How violent!" Doma laughed—a light, crystalline sound that grated like breaking glass. "But that's what I love about you, Akaza-dono! So passionate! So direct! Unlike me, you actually feel all those messy emotions." He pressed a hand to his chest in mock sympathy. "It must be exhausting, carrying around all that anger."
"I said shut up."
Screech.
A third platform rotated into view, grinding against unseen mechanisms. The figure standing there was small, hunched, with horns protruding from a wrinkled head. Hantengu, Upper Moon Four, trembled visibly, his hands clutching at himself.
"S-scary," he whimpered, his voice cracking. "They're fighting again... Why do they have to be so scary?"
Splash
A revolving platform brought forth a bizarre sight—a vase, ornate and grotesque, with writhing flesh visible through ceramic cracks. Upper Moon Five emerged from the vessel like something being born wrong, his upper body twisting free with wet, organic sounds. Gyokko's pale skin gleamed in the lantern light, baby-like hands pressing against the vase rim as mouths scattered across his body opened and closed in unsynchronized breathing.
"Art," he hissed, the word stretching unnaturally. "Lord Muzan's architecture is sublime art. Look at these angles! These impossible geometries!" His main mouth curved into a grotesque smile, revealing needle teeth. "Though I must say, the company leaves much to be desired. Philistines, all of you. No appreciation for aesthetic beauty."
"Nobody cares about your pots, Gyokko," Akaza growled without looking at him.
"POTS?!" Gyokko's voice rose to a shriek, multiple mouths speaking in dissonant chorus. "These are masterworks! Each one a testament to—"
CRASH.
A platform slammed into position with enough force to rattle the surrounding structures. Two figures stood there—one tall and skeletal with black spots across sickly green skin, the other small and beautiful, wrapped in elaborate obi sashes that writhed like living serpents.
Gyutaro hunched forward, his posture twisted and wrong, curved sickles hanging loose in each hand. Yellow eyes, jaundiced and vicious, swept across the assembled demons with barely contained contempt. Black spots mottled his skin like disease, his ribs visible through taut flesh. He scratched at his neck—scritch scritch scritch—the sound grating and constant.
"Tch." He clicked his tongue, the sound wet and harsh. "All the Upper Moons, huh? Must be something big." His lips pulled back in a sneer that showed too many teeth. "About damn time we got some excitement."
Beside him, Daki tossed her white hair, the motion practiced and vain. Her obi sashes coiled around her feet, their edges sharp enough to split stone. Beautiful face arranged in a pout, she crossed her arms beneath her chest.
"I was in the middle of feeding, and some of those girls were particularly tasty." Her green eyes, with the kanji for "Upper Six" written in them, narrowed. "This had better not be a waste of my time."
"Our time, you mean," Gyutaro corrected, scratching harder. Scritch scritch scritch. "We're one demon, remember? You don't get to—"
"I know that, Onii-chan!" Daki stamped her foot, the platform shuddering. "I just meant—"
"Will everyone SHUT UP?!" Akaza's voice cracked across the space like a whip. His blue markings blazed brighter, pulsing with his anger. "Show some damn respect! Lord Muzan could arrive any second!"
"Oh my, is Akaza-dono getting grumpy?" Doma's smile never wavered. "You really should try to relax more. All that tension can't be good for you."
Akaza spun on him, fist raised—
BOOM.
Not a sound—a sensation. Pressure dropping so suddenly that ears popped. The lanterns flickered, their flames shrinking as if afraid. The temperature plummeted, frost spreading across wooden surfaces with soft crck crck crck sounds.
Doma's smile remained, but his posture changed—spine straightening, hands dropping to his sides, all mockery evaporating from his demeanor.
Akaza dropped to one knee instantly, head bowed, pink hair falling forward.
Hantengu prostrated himself fully, whimpering into the wood.
Gyokko retreated into his vase with a wet slurp, only his eyes remaining visible above the rim.
Gyutaro grabbed Daki's head and shoved her down into a bow beside him, his own spine bending as he dropped low. "Don't be stupid," he hissed.
Strum.
A single biwa note—deep, resonant, wrong—vibrated through the impossible space. The sound built, layering upon itself, until reality itself seemed to shudder.
STRUM.
The central platform blazed with sickly purple light.
A woman appeared—materializing as if she'd always been there and they'd simply failed to notice until this moment. Elegant kimono in deep plum, black hair cascading past her shoulders, features sharp and aristocratic. Beautiful. Utterly inhuman.
Muzan Kibutsuji stood in female form, crimson eyes with vertical slits sweeping across the assembled Upper Moons with the clinical interest of someone examining insects pinned to a board.
"Look at me."
Not a request. A command that bypassed conscious thought and seized the nervous system directly.
Heads rose as one, though none dared meet those terrible eyes.
Silence stretched. Frost continued spreading—crck crck crck—crawling up platform supports, coating surfaces in ice that radiated cold like physical malice.
Muzan's gaze moved slowly, deliberately, counting. Doma. Akaza. Hantengu. Gyokko. Gyutaro and Daki.
Her eyes narrowed.
Muzan's head tilted slightly, the movement unnaturally smooth. "Last night, in the forests east of here, Upper Moon One encountered a group traveling toward the Demon Slayer Corps headquarters." She paused "A bunch of humans. And two demons who have broken free of my control."
The words hung in the frozen air.
Doma's rainbow eyes widened fractionally. His ever-present smile stayed fixed, but something in his expression shifted—genuine confusion breaking through his mask. "Lord Muzan, when you say 'encountered'—"
"He engaged them." Muzan's crimson eyes fixed on Doma, and the Upper Moon Two physically recoiled despite himself "He used every bit of his strength—every technique, every ability earned over four centuries. Then "
Gyutaro stopped scratching. His hand froze against his neck, yellow eyes wide.
Daki's mouth opened slightly, her practiced beauty cracking to show genuine fear beneath.
"The connection severed." Muzan's hands flexed, fingers curling into claws. "The blood I gave him—the blood that bound him to me for four hundred years—went silent. And that means-
Crack.
Akaza's platform fractured beneath his knee. His breathing came faster, harsher. His mind raced—Kokushibo, stronger than any demon save Muzan himself, who had slaughtered countless Hashira, who had stood at Muzan's side since the beginning—
"Upper Moon One is dead."
The words dropped like stones into still water, ripples spreading outward in silence.
Doma's smile vanished completely. His face went blank—not emotionless, but shocked into blankness, like a system overloading. "That... that's not possible. Lord Muzan, surely there must be some mistake? Upper Moon One has never been—"
"Are you suggesting I am mistaken, Doma?" Muzan's voice carried no anger, no heat. Just absolute zero cold. "That I cannot feel the death of a demon carrying so much of my blood?"
"N-no! Of course not, Lord Muzan!" Doma's hands came up placatingly, his smile snapping back into place though it looked painted on. "I only meant—who could possibly—"
"The young man." Muzan's kimono patterns seemed to writhe, like living things beneath fabric. "One human. He killed Upper Moon One, all by himself."
Akaza's entire body went rigid. Not from fear—from something else. Something that looked almost like excitement. His fists trembled, blue markings blazing brighter.
"Impossible!" Gyokko's voice screeched from his vase, multiple mouths speaking in discordant harmony. "Upper Moon One was a work of art! Centuries of refinement! No human could—"
"Shut up, you creepy pot-freak," Gyutaro snarled, though his voice carried less venom than usual. His hand resumed scratching—scritch scritch scritch—faster now, more agitated. "If Lord Muzan says it happened, it happened."
Hantengu wept openly, his whimpers echoing off twisted architecture. "We're all going to die! If Upper Moon One fell, what chance do we have? We're all—"
"Silence."
The single word crushed Hantengu's voice like stepping on an insect. The old demon's mouth kept moving, but no sound emerged.
Muzan's gaze moved to each Upper Moon in turn. "This young man possesses a medicine—a cure that can turn demons back into humans. He has used it successfully. Multiple demons have been restored to humanity." Her hands flexed, fingers curling into claws. "The demon slayer boy's sister, who is along with him, was a demon. She is now human again."
Doma processed this, his analytical mind working behind rainbow eyes. "A cure... how fascinating. The medical knowledge required to—"
"I do not care about medical knowledge, anymore" Frost exploded outward from Muzan's platform, coating everything in thick ice. "I care that it exists. That it threatens my existence. That it gives the Demon Slayer Corps hope they have not felt in centuries."
She took a step forward, and the platform groaned under pressure that had nothing to do with physical weight.
"Tamayo travels with this young man."
The name echoed like a curse.
"Tamayo." Muzan repeated it, her voice carrying four hundred years of rage compressed into three syllables. "The traitor who has evaded me for centuries. She aids him. She has likely contributed to this cure." The kimono patterns writhed faster, more violently. "And this human—this ONE human—killed Upper Moon One alone. Not with a group. Not with Hashira support. Alone."
Akaza's breathing had become almost panting. His yellow eyes blazed, fixed on Muzan with intensity that bordered on hunger. A human. Strong enough to kill Upper Moon One. Strong enough to—
"The strongest opponent in existence," Akaza whispered, barely audible.
Muzan's eyes snapped to him. "Yes, Akaza. Stronger than any Hashira, right now" Her lips pulled back from teeth that had gone pointed. "Perhaps strong enough to threaten me."
The admission hung in the air like poison.
Daki made a small sound—not quite a whimper, but close. Gyutaro's hand shot out, gripping her shoulder, pulling her closer. "Quiet," he hissed.
"They travel to the Demon Slayer Corps headquarters now," Muzan continued, her voice dropping lower, colder. "By a few more nights, they will arrive. The demon slayers will learn of the cure. They will learn what this young man can do. They will weaponize both." Her extended hand slowly closed into a shaking fist. "This cannot stand. The threat must be eliminated. The cure must be destroyed. Tamayo must suffer for her betrayal. The demon slayer boy and his sister must die. And this young man—"
The fist tightened until bones audibly creaked.
"—torn apart. Piece by piece. Until nothing remains but screaming meat that begs for death I will not grant."
Doma raised one finger hesitantly, his mask of cheerfulness cracking to show genuine concern beneath. "Lord Muzan, forgive me, but if he killed Upper Moon One, sending more demons might simply—"
"Are you suggesting any of Upper Moons are weaker than Kokushibo?" Muzan's head turned with mechanical precision. Those crimson eyes fixed on Doma, and even with his inability to feel fear, some primal part of him flinched. "That my demons cannot accomplish what he failed to do?"
"Not at all, Lord Muzan!" Doma's words came quickly, smile stretching wider. "I merely thought perhaps a little strategy might—"
"The strategy is simple." Muzan's voice cut through his explanation like a knife through silk. "Akaza."
Akaza's head snapped up, yellow eyes meeting crimson. "Yes, Lord Muzan."
"You live to fight strong opponents. You exist to test yourself against those who might match your power." A cold smile crossed Muzan's features. "If this human truly killed Upper Moon One, then defeating him would prove your superiority, would it not?"
Akaza stood slowly, his entire frame vibrating with barely contained energy. The blue markings across his body pulsed like war drums. "It would, Lord Muzan. I'll kill him. I'll tear him apart and prove that I—"
"You will not go alone."
The words stopped Akaza mid-sentence. His expression shifted from eager to confused to—briefly—angry.
"Doma will accompany you."
"What?!" Akaza spun toward Muzan, shock breaking through his usual deference. "Lord Muzan, please, I don't need—"
"This is not a request, Akaza. It is an order." Muzan's eyes narrowed dangerously. "Upper Moon One was stronger than you. If you engage this human alone, you will die. Doma will ensure the mission succeeds."
"But Lord Muzan—"
"Are you refusing my command?"
The question landed like a physical blow. Akaza's mouth snapped shut, jaw muscles jumping beneath marked skin. His fists clenched so tight that blood welled between his fingers, dripping to the platform with soft plink plink plink sounds.
"...No, Lord Muzan."
"Good." Muzan turned her attention to Doma, who had watched the exchange with fascinated interest. "Doma. You will work with Akaza to eliminate this threat. Do not let your usual... games... interfere with the task."
Doma's smile returned full force, cheerful and empty. "Of course, Lord Muzan! Akaza-dono and I will work splendidly together! Won't we, Akaza-dono?" He turned that smile on Akaza, who looked ready to explode.
"If you get in my way, Doma, I swear I'll—"
"Leave tonight," Muzan interrupted, her voice brooking no argument. "Observe from a distance and Wait for the moment he is separated from the others making him vulnerable. Then kill others and end him as well" Her gaze swept both Upper Moons. "Do not underestimate him and above all, Do not fail like Upper Rank 1"
"Understood, Lord Muzan," they answered in unison, though Akaza's voice carried the enthusiasm of someone being ordered to eat glass.
Muzan's attention shifted to Gyutaro and Daki. "Upper Moon Six."
Gyutaro straightened slightly, sickles tightening in his grip. "Yes Lord Muzan?"
"You will remain in your territory. Continue your work in the Entertainment District." Her crimson eyes narrowed. "If Upper Moon Two and Three fail, I will summon you next."
"Us?" Daki's voice rose in pitch, fear bleeding through her vanity. "But Lord Muzan, if Upper Moon One couldn't—"
"Shut UP!" Gyutaro slapped a hand over her mouth, his yellow eyes wild with panic. "Don't question Lord Muzan, you stupid—"
"I understand your fear," Muzan said, and something in her tone made it clear this was not comfort but observation. "But you will obey regardless. If called, you will answer. If ordered to fight, you will fight. Is this clear?"
"Yes, Lord Muzan," Gyutaro said quickly, still keeping his hand over Daki's mouth even as she struggled against it.
Muzan's gaze moved to Hantengu, who shrank into himself. "Upper Moon Four. You will remain on standby. If needed, you will be deployed." Then to Gyokko. "Upper Moon Five. The same applies to you."
"Of course, Lord Muzan," Gyokko's multiple mouths chorused from within his vase. "Though I must say, I would relish the opportunity to create a masterpiece from this human's corpse. The artistic potential—"
"You will have your opportunity if those above you fail." Muzan's form began to fade, reality peeling away like old paint around her edges. "Remember this, my Upper Moons. Upper Moon One's failure will not be repeated. The next time I summon you—"
The frost cracked louder—CRACK CRACK CRACK—spreading faster, climbing higher.
"—I expect to hear that this young man's head has been separated from his shoulders. That his cure has been burned to ash. That this threat to my existence has been eliminated." Her form grew translucent, fading. "Disappoint me..."
The temperature dropped so low that the ice itself began to crack under the strain.
"...and Upper Moon One will not be the last demon to die."
STRUM.
The biwa note rang out. Muzan vanished like smoke in wind, leaving only lingering cold and the taste of threat hanging in the impossible air.
For several heartbeats, no one moved.
Then Akaza spun on Doma, murder in his yellow eyes. "Stay. Out. Of. My. Way."
"Now now, Akaza-dono!" Doma's cheerfulness returned like flipping a switch. "We're partners now! Teammates! This will be wonderful!" He clasped his hands together. "We can spend quality time together, learn about each other's fighting styles, maybe share some meals—"
"I'll kill you myself before I let you interfere with my fight."
"So hostile!" Doma laughed, the sound like breaking crystal. "But I understand. You want to prove yourself. You want to show Lord Muzan—and yourself—that you're worthy of a higher rank." His rainbow eyes gleamed with something that might have been genuine insight. "This human killed Upper Moon One, so killing him makes you the strongest. Makes you worthy of that position."
Akaza's fists trembled. "That's right. I'll defeat him. I'll prove I'm stronger than he ever was."
"Of course you will! With my help, naturally." Doma waved dismissively. "Though I have to say, I'm quite curious about this human myself. Someone who can kill Upper Moon One in single combat? How fascinating! I wonder what makes him so special? Is it technique? Pure Human power? Some secret weapon?" He tilted his head. "I'd love to talk with him before we kill him. Maybe learn a few things!"
"You're not talking to him, Doma. We're killing him."
"Well yes, obviously!! But that doesn't mean we can't have a civilized conversation first!" Doma's smile widened. "Besides, Akaza-dono, you know how I am. I like to understand my opponents before I devour them. Makes the whole experience so much richer!"
Akaza made a sound that was half growl, half roar. "I don't care about understanding him. I don't care about conversations. I'm going to fight him, defeat him, and prove I'm the strongest. You just stay out of my way and don't interfere."
"We'll see how it goes!" Doma said breezily. "Oh, this is going to be such fun! You and me, working together, hunting down the strongest human in history! It will be so much fun, don't you think?"
"I'm leaving." Akaza turned sharply, preparing to leave. "Be at the rendezvous point after sunset. And Doma—" He looked back, yellow eyes blazing. "If you get between me and this fight, if you rob me of this opponent, Lord Muzan's orders won't save you. I'll rip you apart and enjoy every second of it."
"So passionate! That's what I love about you!" Doma waved cheerfully as Akaza's form began to fade. "See you tonight, Akaza-dono!"
Whoosh.
Akaza vanished, leaving the remaining Upper Moons in uncomfortable silence.
Gyutaro finally removed his hand from Daki's mouth. She immediately whirled on him. "You didn't have to cover my mouth like that, Onii-chan! I wasn't going to—"
"You were absolutely going to say something stupid," Gyutaro interrupted, scratching at his neck again. Scritch scritch scritch. "And get us both killed. You think I'm gonna let you—"
"I'm not stupid!" Daki stamped her foot, obi sashes writhing angrily around her. "I just think it's unfair that we might have to fight someone who killed Upper Moon One! We should get to stay in our district where it's safe and—"
"Nothing's safe anymore, you idiot." Gyutaro's yellow eyes remained fixed on where Muzan had stood. "You heard Lord Muzan. If the others fail, we're next. And if Upper Moon One couldn't beat this human..." He trailed off, scratching harder, faster.
"Then we're dead," Daki finished quietly, her vanity cracking to show the fear beneath. "Onii-chan, I don't want to die. Not when we finally have everything we wanted. Not when—"
"We're not gonna die." Gyutaro grabbed her shoulder, his grip tight enough to bruise if she'd been human. "We've survived this long by being smart. By working together. If it comes to it, we'll figure something out. We always do."
Daki looked up at him, her green eyes wide. "Promise?"
"Promise." The word came out rough, but certain.
Gyokko emerged further from his vase, multiple mouths twisted in disturbed expressions. "Upper Moon One dead. It's unthinkable. The artistic loss alone..." He shuddered, pale flesh rippling. "Four hundred years of technique, gone. Like destroying a masterpiece."
"Nobody cares about your art metaphors," Gyutaro snapped, though his heart wasn't in it.
Hantengu continued whimpering, now that his voice had returned. "We're all going to die. That human is going to kill us all. Just like he killed Upper Moon One. We're all going to die..."
"Oh, shut up, old man," Daki said, her usual bratty tone returning as she tried to reclaim some normalcy. "You're Upper Moon Four. Start acting like it."
"S-scary girl," Hantengu whimpered, curling himself.
Doma stood alone on his platform, that permanent smile still fixed in place. But behind his rainbow eyes, something is calculated. Something cold and analytical.
A human strong enough to kill Upper Moon One.
How absolutely fascinating.
Strum.
The biwa note rang out, and platforms began to rotate, carrying the Upper Moons away into the infinite darkness of the castle.
Gyokko retreated fully into his vase with a wet swish, disappearing from view.
Gyutaro and Daki's platform spun away, the siblings already bickering—her complaining about having to leave her feeding early, him telling her to shut up and focus on survival.
Hantengu's platform descended into shadow, his whimpers fading gradually.
Doma remained a moment longer, still smiling, still calculating.
Then his platform began to move, and he waved cheerfully at the empty air.
"Tonight is going to be interesting!!" he said to no one in particular. "Very interesting indeed~"
STRUM.
The platform vanished, and the Infinity Castle fell silent once more.
In the shadows above, Nakime sat with her biwa, her single visible eye watching everything. Her fingers moved across the strings, adjusting the castle's geometry, keeping the impossible architecture stable.
She said nothing. She rarely did.
But her eye remained fixed on the space where Muzan had stood, and where the news of Upper Moon One's death had fallen like a stone into dark water.
The ripples were only beginning to spread.
. . .
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