One Piece: With Sign-in System

"What am I? Obviously… I'm human."

Ron smiled softly.

And just like that—he vanished from Muzan's sight.

In the very next instant, he appeared right behind Muzan, standing beside Nakime. The crimson blade of his Nichirin sword traced a graceful arc through the air—cleanly slicing across her neck.

Slice—

A head flew.

Muzan's already pale face drained of the last remaining color as he instantly leapt backward, widening the distance between him and Ron. The moment he felt that overwhelming Conqueror's Haki earlier… he understood one terrifying truth—

This man in front of him… was far stronger than that man from four hundred years ago.

No doubt about it.

There was absolutely no way he could win.

There was only one choice—escape.

He had to escape. Otherwise—he'd die.

Driven by pure terror from Ron's Haki, Muzan flash-stepped to the mutilated body of Kokushibo. He transformed his left arm into a spike-tipped tendril, plunging it into Kokushibo's chest, injecting a massive quantity of his own blood. Along with it, he issued a single cold command—

"Buy me time. No matter what—it must be done."

Fueled by Muzan's blood, Kokushibo's body rapidly regenerated a head, then mutated further—razor-sharp blades began sprouting all over his body. His form twisted into something monstrous.

He let out a beastly roar and charged toward Ron with unhinged ferocity.

Moon Breathing, Fourteenth Form: Wild Mutation - Celestial Crescent Frenzy!

Countless massive, blood-red crescent blade winds swept outward, carving through everything in their path as they rained down on Ron from all directions.

At the same time, the Infinity Castle began to collapse.

With Nakime's head turning to ash, the entire space she controlled started to break down.

The moment her head vanished from the world—this alternate dimension would completely disintegrate.

Muzan ran, dashing through the crumbling halls of the Infinity Castle, fully determined—if he could just buy time and get away from that incomprehensible monster, then this time... he would go underground.

No more exposure. No more ambition.

He would hide for a hundred—no, three hundred years.

He would not appear again until that thing died.

As he fled, Muzan's teeth clenched tight. Rage, fear, and unease churned in his chest.

Why?!

Why does a monster like him even exist?!

Where did he come from?!

…

"He's completely turned into a beast now, huh…"

Ron looked at the tidal wave of blood crescents slashing toward him and casually swung his Nichirin blade.

A scarlet ripple of energy burst from his body—tranquil, elegant—and simply disintegrated the incoming storm of blade winds.

Holding the crimson sword in one hand, Ron stared at the grotesquely mutated Kokushibo.

"I originally planned to let Giyu and the others finish you off… but it seems that's no longer an option."

"RAAAAAAHHH!!"

The mutated Kokushibo had long lost his mind. Now, he was nothing more than a mindless beast obeying Muzan's final command.

He leapt high into the air, unleashing a barrage of massive blade winds at Ron—slashes so devastating that even three or four Hashira combined would struggle to survive it.

Ron, however, remained still.

Then, with a single, lazy swipe of his blade—

Whoosh—

A thin, vertical crimson slash tore through the sky, cleanly cutting apart every wave of blade wind and splitting the monstrous Kokushibo right down the center.

"Die under the swordsmanship of the Sun Breathing Style. Before your death—take a good, hard look at the distance between yourself and Yoriichi Tsugikuni."

Ron followed with a second slash—horizontal this time.

Sun Breathing, First Form: Dance of the Fire God.

A blazing arc wrapped in crimson flames burst outward from him, sweeping around in a perfect circle. It carved through Kokushibo's already-split neck, severing both halves of his head in one clean motion.

The circular slash didn't stop there.

It kept going—slicing straight through the collapsing Infinity Castle, cleaving it entirely in half from top to bottom.

From the ruins, Muzan looked back…

And his face twisted in absolute horror.

Nakime's head… had finally crumbled into ash.

The Infinity Castle shattered.

Ron and Kibutsuji Muzan appeared simultaneously in the real world beneath the night sky, the Nichirin blade still in Ron's hand.

Muzan immediately tried to flee.

But before he could make a move, a sharp slash surrounded by flowing blue water came slicing toward him.

It was Water Hashira Giyu Tomioka's Water Breathing, First Form: Water Surface Slash!

Startled like a frightened bird, Muzan was momentarily alarmed by the sudden attack. But when he realized its strength was far below what he feared, he breathed a small sigh of relief and raised his iron-hard whip-arm to block it head-on.

Yet Giyu didn't give him a moment's rest—he followed up with a second technique.

Water Breathing, Tenth Form: Constant Flux!

But Muzan had no desire to fight. His only thought was to escape—escape as far as possible from that monster.

Parrying the next strike with his whip-arm, Muzan continued fleeing in a single direction.

Whoosh!

Two shadows dropped from the sky.

Wind Breathing, Sixth Form: Black Wind Mountain Mist!

Flame Breathing, Fifth Form: Flame Tiger!

Sanemi Shinazugawa, the Wind Hashira, and Rengoku Kyojuro, the Flame Hashira, had just defeated their respective Upper Moons and rushed here without rest. Upon sensing Muzan's aura, they wasted no time. In the same instant, they awakened their Demon Slayer Marks and ignited their bright red blades, slashing without hesitation.

A flicker of fury passed through Muzan's eyes.

These cursed Demon Slayers—of all times, why now?!

Damn them!

His pupils turned crimson in an instant. Thorned whips sprouted from his arms, shattering the two incoming techniques and shooting straight for the Hashira's chests.

Their eyes widened in shock.

Then—

Slash!

Three more figures arrived.

Muichiro Tokito. Mitsuri Kanroji. Shinobu Kocho.

Their blades moved as one, severing the deadly whips before they could land.

Muzan's eyes narrowed sharply.

Boom!

A massive figure crashed down from the sky, slamming into the ground in front of Muzan.

With a heavy chain-linked axe and flail in hand, Demon Slayer Mark etched across his face, Gyomei Himejima stood like a wrathful guardian deity. Gazing calmly at Muzan, he spoke:

"Amitabha. At last, I've found you… Kibutsuji Muzan."

One after another, Obanai Iguro, Tengen Uzui, Tanjiro, Inosuke, and Zenitsu appeared around them, encircling Muzan at the edge of the desolate forest clearing.

Muzan looked around, veins bulging across his pale face, bloodlust erupting.

"Damn the Demon Slayer Corps… damn the Ubuyashiki clan!!"

Tamayo and Yushiro arrived beside Giyu. Tamayo's cold eyes locked onto Muzan as she declared:

"Kibutsuji Muzan, this is the end of your road. Today, you'll die. Repent for the countless lives you've stolen—in Hell."

"Hahaha…"

Muzan burst into maniacal laughter. "Kill me? You think sheer numbers will make it possible?"

"Why not?"

A clear, light voice sounded nearby.

Muzan's laughter halted abruptly.

He turned—and when he saw the tall, poised figure walking over to stand beside Tamayo, his face turned ghostly pale, as if he had swallowed a mouthful of dead flies.

Ron looked at Muzan and smiled. "Relax. I won't be joining in."

"I believe… they're more than enough to kill you."

Muzan didn't know whether to feel relief or despair.

How did it come to this? The mighty King of Demons—brought so low in just one night…

"Die, Muzan Kibutsuji!!"

Sanemi was the first to charge in, followed by Gyomei.

Then, every Hashira joined the fray.

Tanjiro and the rest also entered the battle.

In a single instant, sword techniques bloomed like flowers. Sword aura and slashes filled the air, blades clashing with thorned whips in a fierce, symphonic dance of steel and fury.

Ron sat leisurely atop a nearby hill, a gourd of fine sake in hand, sipping as he watched this destined battle unfold—humanity versus demon, Demon Slayers against the progenitor of all monsters.

Far off in the sky, the horizon began to lighten.

Under the onslaught of the Hashira and their allies, Muzan was slowly forced to exert his full power. His body began to mutate grotesquely—no longer even humanoid. Five brains, seven hearts, countless thorned tendrils and whip-arms… a complete aberration.

In one terrifying moment, the Hashira were pushed to their limits.

But faced with despair, every single Hashira and Tanjiro's group broke through, unleashing power far beyond their previous limits. They turned the tide in a single, unified counterattack.

Meanwhile, Tamayo's drug had begun to take effect. Muzan's body could no longer maintain its form—his strength was rapidly decaying.

The Hashira seized this opportunity.

One after another, they disabled his most dangerous weapons.

Gyomei shattered his skull with a single mighty blow.

Rengoku and Sanemi sliced off the whip-arms growing from his hands.

Shinobu, Mitsuri, and Obanai pierced three of his hearts.

Muichiro severed five thorned tendrils.

Zenitsu, Inosuke, Kanao, and Genya combined forces to destroy the remaining four.

"It's over, Muzan!"

With the ability to see the Transparent World awakened—and having realized the Thirteenth Form of Sun Breathing in the midst of battle—Tanjiro and Giyu struck together.

Their blades cleaved through Muzan's neck, stabbing through three of his hearts.

Only one remained.

Cornered at last, Muzan unleashed his final form:

The Infant State.

Layers upon layers of regenerating flesh enveloped his core, shielding his last heart as he began stumbling away, ignoring the blades cutting into him, trying to flee in a single direction.

But dawn was breaking.

And every Hashira still stood, blades in hand.

As the sun rose in the east—

Muzan was consumed by the light, vanishing into ash.

The battle was over.

Ron tossed aside the empty sake flask and slowly rose to his feet.

With one hand on his waist, he looked at the exhausted warriors collapsing into the grass and smiled softly.

"What a fine battle this was…"

Shinobu seemed to hear his voice. She turned her head to look.

That tall, composed figure raised a hand in farewell, smiled, and said, "Good work, everyone."

Then, he vanished into the morning light.

…

Upon the hill…

Only a single Nichirin blade remained, stabbed into the ground.

And hanging from its hilt—was a vial of medicine.

"…Farewell, Mr. Ron."

Shinobu whispered, tears slipping silently down her cheeks.

Bright golden sunlight bathed the land.

And from that day forward—

There were no more demons in this world.

- - -

- [End of Demon Slayer Arc] -