Sign-In System: One Piece, Demon Slayer & Beyond

Snow blanketed the mountains, turning the entire world into a vast stretch of white.

Deep within a desolate mountain forest—

A lone rabbit foraged for food beneath the thick snow.

Suddenly—

In the clearing, the air above twisted violently, forming a spiraling distortion about five meters wide.

A figure, drenched in blood, dropped out of the swirling vortex and crashed into the snow with a heavy thud.

Startled by the sound, the rabbit bolted in panic, darting into a nearby burrow.

Time passed.

Sensing no further danger, the rabbit cautiously emerged again and resumed its search for food in the quiet clearing.

Amidst the untouched snow, the blood-soaked figure of a young man lay still, gripping a broken blade in his hand.

No one could have imagined the immense vitality that surged within that battered body, hidden beneath the gruesome, blood-drenched wounds—

Wounds that were rapidly healing at an inhuman pace.

As night fell—

Moonlight draped across the snow-laden forest, casting a cold, tranquil glow.

"Blood… I smell blood…"

"Over there…"

From the shadows, a swift and ghostly figure flashed through the trees.

It landed silently in the blood-stained clearing. The moonlight poured over its form, revealing a monstrous silhouette that stepped out of the darkness.

It looked human—head, arms, legs—but the features were twisted and grotesque. Especially the eyes—slitted like a beast's, gleaming with savage, unnatural bloodlust.

It was no human.

It was a demon—

One of the creatures that prowled the hidden darkness of this world!

"I found it…"

"The smell of blood… such a sweet aroma…"

Greed and hunger burned in its eyes as it approached the young man lying in the snow.

It had devoured many humans before—young and old, men and women—but never had it encountered such an intoxicating scent of flesh and blood.

"If I eat him… I'll definitely grow stronger…"

Salivating at the thought, the demon licked its lips with a long, serpentine tongue.

Unable to hold back any longer, it stomped the ground, powerful legs launching it into the air. With jaws wide open and teeth glinting like knives under the moonlight, it dove at the seemingly unconscious man.

Just as those razor-sharp fangs were about to bite down—

The young man opened his eyes.

Before the demon could even register what had happened, an explosive force surged from the man's body and sent it flying backward.

It slammed into a tree as thick as three men's arms stretched wide, snapping it clean in half.

Snow rained down from the treetop and buried the demon beneath.

Groaning, the demon clawed its way out of the snow and looked up.

The young man now stood tall, clad in a long, black coat draped over his shoulders, his upper body bare and slick with blood.

He stood over two meters tall, with long, wild black hair cascading down his back. His features were strikingly handsome, and his frame was lean yet powerful. Every muscle on his exposed torso was chiseled to perfection, radiating a strength that went far beyond human limits.

The demon, whose strength was already approaching that of a Lower Rank Six, stared at the man—

And for the first time, a deep, primal fear gripped its heart.

A fear born from instinct!

The man glanced at the broken blade in his hand, then slowly scanned his surroundings.

Finally, his eyes fell on the demon.

In that instant—

A crushing aura surged forth like a tidal wave!

"—!"

Under the pressure of that terrifying presence, the very air around the demon thickened and froze.

It was paralyzed.

Despite possessing a body far beyond a normal human's—capable of tearing people apart like paper—

The demon couldn't move a single finger under that overwhelming pressure!

"…This place… doesn't look like the world of pirates anymore. Tch. That's gonna be a problem."

The young man tilted his head back and gazed at the moon, muttering softly to himself.

He didn't even spare the demon a second glance.

That demon—who could easily tear through entire villages—

Now found itself crushed under the sheer force of the man's presence, trembling, eyes wide in horror.

What kind of monstrous aura is this?!

Is he… one of the Hashira?! No! Even a Hashira doesn't have an aura like this!

Just what the hell is he?!

As if hearing its thoughts, the man turned to look at it again, his expression calm and cold.

"You… eat people, don't you?"

The demon met his gaze—

And when it saw the loathing and disgust in the man's eyes, a bone-deep chill swept through its entire body.

It felt like falling into an icy abyss.

The next second—

The young man raised his broken blade.

In the demon's eyes, filled with despair and terror, he gave a casual swipe.

The earth trembled.

As the smoke and snow settled, a massive trench now scarred the land—

A hundred meters long, and ten meters deep.

The demon was gone. Not a single trace remained. Not even a cell.

As if he had merely brushed away a speck of dust, the young man tossed aside the shattered blade, his expression calm, almost bored.

He looked up again at the moon hanging high in the sky, his brows faintly furrowed.

"Didn't think getting caught in that time-space storm would fling me into another world entirely… Now the real question is—how do I get back?"

That young man—

Was Ron.

And as for why he had appeared here, it all began with that battle…

The battle in Mary Geoise.

During that final battle between the two sovereigns—him and Imu—

Just when Imu was about to be obliterated by his most powerful attack, the Golden Wheel Reincarnation Burst, she twisted time itself with the last of her strength. In a desperate bid to reverse the clock by just a few seconds, she tore open the laws of reality.

But Ron, already at the brink of his limits, poured everything he had left into a final move—Lv3 Ultimate Kick.

With it, he destroyed Imu completely.

Yet in doing so, he himself was caught in the chaotic and twisted time-space vortex that her powers had triggered.

In the instant before he vanished—

He caught a glimpse of Kuina and Hancock, their faces filled with panic and worry.

He said just one last thing to them.

"Wait for me to come back."

Then he was swallowed whole by that shattered, swirling void of space and time.

—And when he next opened his eyes, he had already arrived in this world.

…

As these memories came flooding back to him, Ron let out a long sigh, still gazing at the pale moon high above.

"System, is there any way to get me back?", he asked inwardly.

But several seconds passed, and the system remained completely silent.

Ron's brow furrowed.

"…System?"

Just then, the familiar mechanical voice finally rang out in his mind.

[Ding! Host has been detected in another world.]

[Gift Received: Ice-Type Imperial Arm]

[Initiating upgrade for the Strongest Sign-In System.]

"Huh?" Ron blinked.

[All functions of the system will be disabled during the upgrade. System space will become inaccessible.]

[Estimated upgrade duration: 30 days.]

[System upgrade will begin in 10 seconds.]

[10… 9…]

"Wait—wait a second!"

As the robotic countdown echoed in his mind,

Ron didn't waste a single moment. He instantly opened the system space and began pulling out whatever he could at lightning speed.

[…2, 1. Ding! The Strongest Sign-In System is now upgrading…]

After that final notification, The system went completely silent. There was no more sound, no more presence—nothing.

"…You seriously chose now of all times to pull this crap? Dammit!"

Ron's rare flash of frustration burst out. He stomped down hard on the snowy ground. 

An explosive force erupted from beneath his foot, and the land around him—spanning dozens of meters—sank inwards violently, layer after layer of snow and earth crumbling outward.

It was a stomp of pure frustration.

And once it was over—

Ron slowly calmed down.

Being angry was useless.

No matter how furious he got, it wouldn't take him back to the world of One Piece.

If he wanted to return, His only hope now rested in the upgraded system.

"Thirty days, huh… No choice but to wait it out."

With a breath of composure, Ron looked down at the items he'd managed to grab in those final moments before the system sealed itself.

-> The Endless Flask of Wine

-> A change of clothes

-> A silver revolver with infinite bullets

-> A handful of gold coins

That was all he had time to snatch up before the system shut down.

Some of the really valuable stuff?

He didn't even get the chance to touch.

But—

Ron shrugged lightly.

"Eh, whatever."

Ron lifted the Endless Flask, and took a swig.

As the rich, mellow liquor slid down his throat and warmed his gut, the lingering unease in his chest—the disorientation of arriving in an unfamiliar world—finally began to fade.

"Phew…"

He exhaled softly, releasing a faint mist of alcohol into the cold night air.

Then he stood up.

Without hesitation, he stripped off his tattered, bloodstained clothes and changed into fresh ones.

As he dressed, he also carefully assessed the state of his body.

"Too much ocular power from the Tenseigan has been consumed. I'll need time before I can use it again."

"The floating ability from the upgraded Float-Float Fruit—no issues. Fully functional in this world."

"This Ice-type Imperial Arm looks good too. Cool."

"All three types of Haki are available as well."

"Sun Breathing is still operating continuously, but my damaged body hasn't recovered to its peak. I'm probably at… thirty percent at most."

"Swordsmanship? No problem executing the techniques—but every single one of my treasured blades was destroyed in the battle with Imu…"

While running through this mental checklist, Ron fastened his clothes with practiced ease.

Then, his gaze turned toward the massive ravine he'd carved earlier with a single slash.

"So this world has man-eating monsters too? But that one was weak. Is it just that he was weak… or is the power level of this whole world inherently low?"

"Either way, first step is to figure out what kind of place this is."

Fully dressed now, Ron tied back his long hair into a sleek ponytail, the fresh outfit making him look sharp and battle-ready.

He hooked the Endless Flask at his hip, tucked the Silver Revolver with infinite ammo into his coat, and slipped the small pouch of gold coins into his sleeve.

Once everything was in order, He stepped forward into the snowy mountain forest, heading in a certain direction.

Before long, His figure had vanished into the depths of the forest.

Behind him, a trail of footprints marked his path across the snow—Footprints that led back to a mountain slope that had been annihilated by a single swing of his sword.

And lying silently in that snow-covered battlefield—

Was the shattered blade of a once-legendary sword from the One Piece world.

Under the cold light of the moon, Its broken edge, still attached to the hilt, glimmered faintly— Reflecting a silver-white gleam, like frozen starlight.

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