INFINITY CASTLE(just before the collapse of Infinity Castle)
GENYA'S POV
We were dragged here—forcefully transported into this twisted hell called the Infinity Castle.
The air reeked of blood and madness. Around me, Demon Slayers fought tooth and nail, blades flashing, screams echoing off the warped, shifting walls. At first, panic gripped everyone. The overwhelming number of demons, the suffocating aura of despair—it almost crushed them.
But then… the machines came.
Alex-san's mechanical humanoid golems descended like divine wrath—towering constructs of steel and arcane tech. They moved perfectly, cleaving through demons like paper, shielding us with unflinching efficiency.
That… changed everything.
Morale surged.
The tide began to turn.
As for me, Alex-san entrusted me with a sword and a gun.
The sword… it felt like a miracle. I could cleave through demons I couldn't even scratch before. The blade hummed in my grip, pulsing with an almost alive energy.
But the gun… that confused me.
It was so small. So unassuming. I remember holding it up, frowning, then waving it toward Alex-san—not exactly pointing it at him, but I wasn't being careful either.
"Why is this thing so tiny? You expect this to help me kill someone like Nakime?"
His eyes widened, and he took a step back.
"Never do that again," he said sharply, tension in his voice.
Still, he didn't take the gun back.
"This will help you complete your mission," he told me.
My mission…
To kill Nakime.
The Biwa demon.
The one who controls this cursed hellscape.
Alex-san was clear with his instructions:
"Only strike when Muzan is away from her. Not before. If you miss the moment, don't fight—just survive. Well, it will only happen when you are transported to the castle in the first place."
He trained me himself for this task—using his shadow clone what he called. And also taught me breathing style but I don't know why it's called breathing style I just have to trigger the gun.
I still can't use Breathing Techniques. My body just doesn't align with it—at least, not properly.
Even though the healing potion Alex-san gave me helped me tap into real breathing technique but… I'd need at least another year to actually use it in combat.
But I'm not alone.
The humanoid golems fight alongside me, synchronized and ruthless. They fill the gaps where I falter. It's like Alex-san planned this out perfectly—he always does.
Just as I was thinking all this…
BOOOOM!
A deafening shockwave echoed through the castle, as something massive had crashed down from the heavens.
I turned toward the source, my heart pounding.
Below us, the golems' optics flared as they scanned in all directions. Then, one of them spoke, its voice cold and exact:
"Target: Muzan has been relocated. Nakime the biwa demon is currently unguarded. She remains the control node of the Infinity Castle."
My breath caught.
This was it. The moment.
"No Muzan?" I said, already pulling out the gun from my side holster.
"Then take me to her. If I just aim at her… that should be enough, right?"
Alex-san's voice echoed in my mind—his calm, calculated instructions from our final briefing:
"You don't need to kill her with strength. Just aim. Let the weapon do the rest."
The golems acknowledged my command.
"Launching: Genya, Demon Slayer ID 17," the Spade Unit confirmed.
Metal arms snapped out, gripping me by the waist and shoulders.
"Wait, wait—what do you mean by—"
FWOOOSH!
They launched me.
I shot through the air like a fireworks rocket, wind screaming in my ears. My eyes locked on the platform ahead—Nakime, poised and composed, her fingers gently strumming her biwa as the castle twisted and contorted beneath her.
But for once, she was alone.
This was it.
As I soared through the air like a damn cannonball, I locked eyes on my target—Nakime, the Biwa demon. She stood on a floating platform far below, her fingers gently plucking at the strings, manipulating the monstrous labyrinth of the Infinity Castle.
She paused, her head tilting—she sensed something.
But she was too late.
"American Hood Breathing: First Form – Glock 18!" I screamed with every ounce of my lungs, triggering the gun with theatrical fury.
The sound echoed like thunder through the castle's hollow chambers.
Elsewhere, mid-battle—
Sanemi's eye twitched.
Tanjiro stumbled mid-air, baffled.
Giyu's expression actually changed for once—pure confusion.
Cara, standing atop a floating stone with Jogoro and two Spade Units, slowly raised a hand to her forehead and facepalmed hard.
"Of course, it's Alex's influence… I should've guessed," she said as if given up on his antics
Jogoro tilted his head, scratching his chin thoughtfully. "American Breathing? I've… never heard of that style before."
Behind him, the Spade Units silently recorded the entire moment—multiple angles, high-definition. This footage was absolutely going into their "For Master Alex" highlight reel.
Back to Genya—
I pulled the trigger on the tiny gun. It was barely larger than three fingers, looking more like a cheap toy you'd win at a festival booth. But when I fired—
CRACK!
The air split open with a sonic boom. The recoil sent me flying backward, spinning mid-air like a ragdoll. My arms went numb instantly.
And then…
BOOOOOOM!
CREAAAAAAK!
The shot obliterated Nakime's entire platform in a single instant. The force ripped through the air like divine wrath, sending shockwaves that shattered stone and warped space.
IMAGE OF THE GUN
(an: it uses solar energy and can only be used one time a day it can't be used on Muzan because he can escape in disguise that's why Alex didn't use it on him)
One second she was there—
The next, she was gone.
Vanished. Erased. Just like that.
I blinked in midair, looking down at the smoking toy-sized weapon in my hand. "What the hell…" I muttered.
A wild laugh bubbled out of me as I tumbled through the air.
"So that's why Alex-san was scared of this little thing..." I chuckled, my chest heaving with adrenaline.
"It looks like a toy from a matsuri stall—but it just deleted a demon."
With Nakime gone, the Infinity Castle began to collapse.
The walls warped inward, twisting upon themselves. Floors shattered, and gravity bent. Portals opened across the space as reality itself unraveled. The spell was broken. The control was gone.
All who were in Infinity Castle teleported to the surface, which was the same village where Alex and the others had just finished their fighting.
``````````````````````````
UNKNOWN VILLAGE
GENERAL POV
The last of the demons had fallen.
From Daki and Gyutaro to the weakest stragglers still clawing for life, all had been purged. Their end may have been anticlimactic, their death not marked with drama or grandeur… but still, I bowed my head slightly to the duo of brother and sister.
May you finally rest in peace.
As the village recovered from the recent battle, we gathered, preparing to send a strike team into the Infinity Castle.
Alex stood at the center, giving out instructions. His ability to teleport others made him a natural leader. But before any decision could be made
CREAAAAK—BOOOOM!
A thunderous noise erupted from nowhere. A tear split through the sky above us, and one by one, like ragdolls flung from the sky, the Demon Slayers came crashing down into the village square.
Smoke. Screams. Relief.
Alex's grin stretched from ear to ear.
"Hah! They did it! They actually fucking did it on their own!" he laughed, pride glinting in his eyes.
"That training wasn't wasted after all…"
Around them, Slayers collapsed from exhaustion, while Spade Units scrambled to catch those falling from greater heights, cushioning the wounded with their reinforced arms.
"Rescue units—start treating the critically injured!" Alex shouted over the din.
"All Slayers who are tired or wounded—fall back! Muzan is still alive, and Upper Moon One might still be in play as well. This is it (Alex doesn't know Kokoshibo is already dead). The final showdown."
Then—
CRACK—!!!
Something slammed into the earth from above, creating a crater and kicking up a cloud of dust thick enough to blot out the sun. Silence fell.
Alex's eyes narrowed.
From the smoke emerged the figure of Muzan Kibutsuji, battered but alive, his aura crackling with malevolence and rage.
"So," Alex called out coldly, "High and mighty Demon King—how does it feel? Getting your ass handed to you by the very Slayers you once called pests?"
Muzan snarled, his pride shredded. "You bastard—once I kill you with my own hands, I'll—"
SWOOSH—!
Before he could finish, Alex vanished and reappeared right in front of him. Without warning, he grabbed Muzan by the head and dragged him away from the group like Ichigo yanking Aizen mid-monologue, not even entertaining the idea of listening to another villainous speech.
"Now you'll feel what true helplessness is," Alex said without even looking at him.
And with that—
SPLURCH!
Alex ripped Muzan's head clean off, spine and all, in one brutal, fluid motion. Horror flickered in Muzan's crimson eyes.
His body, headless, staggered—then began to regenerate.
Alex didn't even blink.
"Sun Breathing—Fourteenth Form: Solar Thrust."
This was a technique Alex invented himself—crafted specifically for abominations like Muzan, creatures with multiple brains and hearts.
In this form, the user channels lightning-infused chakra into the blade and then thrusts the sword into the enemy. The moment contact is made
—the chakra detonates, incinerating from the inside out.
SLASH. PIERCE. EXPLODE.
In a heartbeat, Alex pierced Muzan eleven times—seven hearts, four brains—gone. Vaporized in a burst of fiery chakra, each one reduced to cinders.
Muzan screamed.
"WAAAAHHHHHHHHHHHHH!!!"
The sole brain that regenerated felt the full pain of the others being obliterated. The agony was unspeakable. Like being burned alive from within, organs exploding like molten iron, and every pulse of chakra reactivating the torment.
The blade trembled with violent energy as the kinetic force was absorbed into the sword.
Alex roared—
"HAAAHHH!!!"
—and swung the katana with every ounce of his enhanced strength, which now exceeded 150 tons, amplified by his Minato template's chakra reserve which was increased with the assimilation.
BOOOOOOOOOOM!!!
The swing collided with Muzan's torso, blasting away trees, homes, and debris in a massive shockwaveAs Muzan flew away, he crashed down onto the trees and then onto a house. Muzan's entire right side was obliterated, his body now hanging together by a single sinew of flesh on the left.
Like Toji's half-ruined corpse… but more grotesque.
Alex rested the katana on his shoulder.
"So... shall I kneel before you now, Muzan-sama?" he asked mockingly. "Like your pathetic Lower Moons used to?"
Muzan, coughing violently, trembled. Memories of helplessness returned—the same fear he once felt before another man. Now, it was happening again.
He considered running.
But…
Muzan Kibutsuji's body trembled, rage and desperation melding into a single terrifying will.
Nezuko…
With her, I can finally walk under the sun. I'll be the perfect being… No one will be able to stop me.
His jaw clenched.
One more shot… Just one. If this doesn't work… I'll run.
Without hesitation, Muzan plunged his clawed hand into his own chest, veins bulging across his torso as black blood surged with unnatural violence. He instantly regenerated but with a catch.
His short black hair elongated into flowing white strands, a twisted halo of corruption. His limbs mutated grotesquely—jaws full of teeth sprouted across his arms and legs, slavering and gnashing like ravenous beasts.
He smiled—wide, unhinged.
"Once you're down… everyone else dies next," he growled, his voice thick with monstrous delight.
From his back, tentacle-like flesh sprouted, writhing like serpents in heat, eager to tear the world apart.
Alex didn't even flinch.
He tilted his head, mockingly unimpressed.
"So that's it? Using your own life force for this final form?"
"I thought you might at least try poison or illusions, something creative. But this? Kekeke… You really thought brute force could defeat me?"
"Idiot."
Muzan's muscles tensed. His jaw ground so tight, that his fangs scraped together. Fury flooded his body, and in a flash—he struck.
SHRIIIEEEEEK—!
Tentacles lashed out in a chaotic storm, dozens of them, thick and sinewy, sharp enough to slice stone and steel. But Alex stood firm, eyes blazing.
"I won't make the same mistake Yoriichi made."
He raised his katana high above his head.
"Sun Breathing – Fifteenth Form: Crimson Sword."
The blade ignited with roaring flames, the middle of the blade shimmered with purple energy, shaped like a vertical slit… an eye, watching, judging.
Then Alex vanished.
BOOOOM—WHOOSH—!!!
He moved at five times the speed of sound, a blur of fire and steel, slashing relentlessly. Every motion burned through Muzan's tentacles, slicing and incinerating them mid-air.
SLASH. SLASH. SLASH!
With surgical precision, Alex severed Muzan's right arm—then his left leg—then the other, the blade singing with destructive intent. The aura around the sword was unnatural—pure sword intent, cutting not just flesh but willpower, unraveling his regeneration like threads.
Muzan screamed in agony.
"AAAAGHHHHHHH!"
He flailed as his limbs vanished, unable to regenerate them. His healing factor, once invincible, was failing. Something else—something white and foreign—was crawling across his wounds, devouring him like acid.
He knew.
If that white energy reaches my head... if it severs it—
Even the sun won't be necessary. I'll die.
He was shaking. Terrified.
Then… a whisper.
"Now you're afraid."
That voice chilled him to his core.
And before Muzan could respond—
SHING!
His vision twisted violently. His body spun away. The world flipped upside down.
He realized.
That's my body…
His eyes, still blinking in horror, stared down at his own decapitated body, still being devoured by that eerie white energy. Slowly. Painfully. Every inch of it burned like hellfire.
His mind was screaming—but then the scene changed.
No pain.
No battlefield.
Just… heat.
He found himself somewhere else.
A massive volcano. Lava churned beneath his feet. The sky glowed red. The air was unbreathable.
"W-Where is this?! Why?! I'm already dying! Why bring me here?!"
Muzan turned, his voice ragged, panicked.
Floating above him was Alex, completely calm, his body wrapped in swirling green energy, radiant and divine.
Alex stared down at him.
"You know…" he said, tilting his head, "Just for fun."
And then—
He flung Muzan's head and body into the volcano.
The flesh hissed. A horrible screech echoed across the volcanic plain as Muzan's last scream was swallowed by molten death.
Alex hovered silently for a moment.
Then he opened his system panel.
[MISSION CLEARED]
Objective: Eliminate Muzan Kibutsuji and his Kizuki – COMPLETE.
Alex let out a yawn.
"Well… I wanna sleep now. Owaaah~…"
With that, he vanished.
`````````````````````````````
UNKNOWN VILLAGE
ALEX'S POV
I reappeared at the same place—the ruined village where we had brought down Upper Moons Two, Four, and Six. The battlefield still bore scars of the chaos that had unfolded.
Craters smoldered, broken tiles and shattered wood littered the streets, but amid the devastation… there was movement. Life.
Demon Slayers, though bruised and battered, were working tirelessly. The Spade Rescue Unit coordinated with the Attack Divisions, using their nano-healing technology and plasma sensors to detect internal bleeding, punctured lungs, fractured bones—anything that could be fatal.
The air smelled of ash, blood… and relief.
As soon as I arrived—still hovering in the sky with green energy swirling around me like a halo of justice—everyone below turned.
The moment they sensed my presence, all eyes locked on me.
From the Hashira, standing tall despite their wounds… to the Ubuyashiki family, watching with serene awe… to Nezuko and Tamayo, standing beside the medics… even the most exhausted Slayers raised their heads.
They looked up at me… like I was something divine.
I met their eyes… and smiled.
A smile that wasn't just for them—it was for the future they fought for.
"ALL OF YOU—MUZAN IS NO MORE!" I called out, my voice amplified through the air like thunder.
"THIS DAWN—WILL BE THE FIRST DAWN WHERE NO DEMONS EXIST BENEATH THIS SKY."
"FROM THIS DAY FORWARD, NO FAMILY WILL BE SLAUGHTERED IN THE NIGHT!"
For a heartbeat, silence.
Then the roar came.
"YEAH!!!"
"OOOOOHHHHHH!!!"
Fists raised. Swords clashed in the air. Tears poured freely. Some fell to their knees and cried. Others laughed like maniacs finally freed from a lifetime of darkness.
The celebration didn't last long—everyone was still tired—but that moment… it was pure.
As the cheers quieted, I descended gently from the sky, the green aura still flickering like an emerald flame.
"Alex-san… are you really human?" Kagaya Ubuyashiki asked, stepping forward with gentle awe in his eyes.
He looked fragile, yet peaceful. His smile was soft, yet proud. He gazed up at me—not as a leader, but as a man simply grateful.
I chuckled.
"It's not what you think," I said with a grin. "I'm human. I just… have powers and abilities far beyond normal people."
I met his gaze steadily, making sure he understood. I didn't want the look of fanaticism. I wasn't a god. I didn't want to be worshipped.
Just trusted.
That was enough.
I turned as I sensed a familiar presence.
"Ah… Tamayo," I called out. "Why are you outside? The sun's about to rise—"
Then I paused. I saw it.
They were changed.
Tamayo, and her ever-loyal assistant Yushiro—both of them had already transformed into humans again.
Tamayo smiled softly, brushing her hair from her face as she walked toward me, the morning light grazing her skin.
"It was… difficult," she said quietly. "Becoming a human again wasn't easy. But thanks to your potion… we were able to suppress the pain but it could not suppress fully."
She reached over and gently patted Nezuko's head, who stood proudly beside her—now fully human once again. Her golden eyes sparkled as she leaned into Tamayo's touch.
Yushiro, meanwhile, gave Nezuko a glare so sharp it could cut through steel. The poor guy looked like he'd just lost his favorite seat at a festival.
I let out a small laugh.
The pain she mentioned... I knew it.
Tamayo and Yushiro, unlike Nezuko, had eaten humans during their time as demons. The cure fought their bodies hard, rejecting the change. It hurt them.
But even though the rejection, they endured. The transformation held.
Nezuko, who had never consumed human flesh… she had it easier. No backlash. No agony. She had turned back peacefully, like the sunrise itself.
I exhaled slowly, watching the light spread across the shattered village.
A new dawn had begun.
Just as I was talking with Kagaya and the others, two familiar presences crashed down from behind me like twin bolts of emotion.
Shinobu and Cara—bloodied, bruised… but alive.
Before I could fully turn, they lunged at me.
But I had already sensed them approaching with my heightened senses.
I caught them in my arms, wrapping them both in a tight, grounding hug.
Shinobu clung to me, trembling.
"Thank you, Alex… I was able to achieve the one goal I had set for my life…"
"And I didn't have to sacrifice myself to do it."
Tears streamed silently down her cheeks, but she wasn't in pain. Not anymore.
Her expression radiated freedom. The weight she'd carried… was finally gone.
"You don't have to thank me," I whispered, gently brushing my thumb along her cheek.
Without another word, I leaned in and placed a soft kiss on her forehead—just above the line of her bangs.
Cara, standing right beside her, immediately gave me a hopeful look, her lips slightly pouting as if to say, "What about me?"
I smirked—and kissed her forehead too.
"We'll be heading off to take care of some… personal things," I said over my shoulder to Kagaya.
And in the blink of an eye—I vanished with both Shinobu and Cara.
``````````````````````````
BUTTERFLY ESTATE
7 HOURS LATER – 12:30 pm
THIRD PERSON POV
"HAHAHAHAHAHA!"
Alex was rolling across the wooden floor near the open window, laughing like a man possessed by pure joy. The moonlight streamed in, illuminating clothes scattered all over the room—as if a mini storm had passed through.
In the center of the chaos, Shinobu and Cara lay fast asleep, tangled in soft silk sheets, both completely bare and resting peacefully. Alex, meanwhile, had slipped into a light kimono, still barefoot and wild-haired.
Across from him on the window frame, a crow wearing a purple scarf stood sternly—looking absolutely done with life.
"One more time!" Alex gasped, trying to catch his breath between laughs. "Please, one more time!"
The crow rubbed its head with a wing, exasperated.
"Alex-sama… you always say that. Every time, you swear it's the last time, and then—this happens again. You have serious issues."
Alex just grinned like a child caught red-handed stealing sweets.
"C'mon… say it. I'm thinking of taking you as my reward from Kagaya. You'd make a nice souvenir."
"Please don't," the crow groaned. "I'll say it… but then you need to go to the Ubuyashiki estate in one hour. The master wants to see you. Okay? I'll say it one last time."
The crow took a dramatic pause.
Then, with a mighty spread of its wings, it spoke:
"Yokoso, Watashi no Sōru Sōsae-ti e!"
Alex exploded with laughter again, doubling over.
"BAHAHAH! I can't—I seriously want Aizen to meet you one day!" he wheezed.
The crow flapped indignantly.
At that point, Shinobu stirred, her delicate form stretching beneath the sheet before she sat up, eyes still half-lidded.
"Love, don't bully General Kasugai Crow…" she murmured, crawling to Alex and wrapping her arms around him from behind. "He always looks so dejected after you mess with him…"
Alex leaned back into her embrace, still giggling softly.
"You'll understand one day. Just wait until you meet him."
"Him?" she asked, puzzled.
He turned his head slightly, meeting her violet eyes with a mysterious glint in his.
"I'll tell you when the time comes. Didn't Cara tell you… everything?"
She nodded slowly."Yes… she did. But it's still hard to believe."
He gently held her hands that were wrapped around his chest.
"It's okay. You will understand."
Unbeknownst to the two, the crow was still there—hovering awkwardly.
"Uh… well, I wasn't spying," the crow muttered. "But the last time I left without telling you, you captured me and gave me to Inosuke…"
Before either of them could respond, the crow beat his wings and vanished into the night sky, still grumbling about his fate.
```````````````````````````
AUTHOR'S NOTE: That's how the finale BATTLE of DMEON SLAYER ended, even though it was rushed. As of now, the Demon Slayer arc contains more than 48000 words, and there will be one more chapter, bringing the total to over 54000 words. Overall, it has been quite long for a sub-arc of the fanfic.
Some would argue that we never really see those Justus. First of all, Alex is already an enhanced individual. With Sun Breathing and the Chakra Cloak with a level of jonin, he can easily become a powerhouse with sheer raw strength exceeding 100 tons, so he doesn't need any fancy jutsu.
If you had read the manga, you would know how Yorichi easily overpowered Muzan. Alex, who is about SEVERAL times more powerful than Yorichi, would find defeating Muzan as easy as beating a child. However, let's not think of it in those terms!
In battle against Akaza, it was his first real fight, so he was quite tense.