Crossover: The Problematic Deity of Arcadia

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Fear.

Kibutsuji Muzan was absolutely consumed by it.

Another man had appeared—one that he couldn't handle. The last time he felt this kind of helplessness was five hundred years ago, when the strongest man in history, Yoriichi Tsugikuni, nearly ended him.

But this time… it felt even worse.

From what Muzan had seen in that final moment before Rui died, this man—whoever he was—was more terrifying than Yoriichi. At least with Yoriichi, Muzan could barely follow his movements. This guy? Muzan couldn't even see them. It was like he wasn't even moving within the same reality.

This was worse than Yoriichi coming back from the dead.

"Those horns on his head… is that guy a demon too?"

Pacing nervously around the room, Muzan muttered to himself, his face pale.

"No… that's impossible. All demons come from my blood. There's no way a demon could exist without me knowing."

That, at least, Muzan was certain of.

The man who killed Rui wasn't a demon. Sure, he had horns, but that could be just a weird accessory—or maybe he was one of those rare humans who gained power by eating demons.

Yeah, those freaks existed too. Rare, but real.

"Should I send all the Upper Kizuki to gang up on him?"

The moment Muzan said that out loud, he shook his head furiously.

"No. Bad idea. If I gather all of them in one place, what if they decide to gang up on me instead?"

That's right.

Muzan didn't trust the Upper Kizuki.

Even though he made them himself—even though he embedded his blood and curse into their very beings, giving him control over life and death—he still didn't trust them. In fact, that made him trust them even less.

Normally, Muzan would never allow the Twelve Kizuki to meet up or even interact. If two of them got too close, he would instantly start monitoring them like a hawk, just in case they tried something.

He wasn't about to make the same mistake he made two centuries ago, when Tamayo rewired her own brain and broke free of his control.

Never again. He wouldn't give any demon the chance.

But even beyond all that…

"Even if I sent all of the Upper Kizuki at him… could they even kill that guy?"

Yoriichi had nearly ended Muzan in a single move. The Twelve Kizuki couldn't even touch Yoriichi. So what chance would they have against someone even stronger than him?

And Muzan? Fight him himself?

Yoriichi nearly erased him in one blow. That wound still hadn't fully healed—and it'd been five hundred years!

Face someone worse than Yoriichi? You've gotta be kidding.

"There's only one option left…"

Muzan lowered his hat brim, eyes filled with icy resolve.

He left the house in Edo where he had been hiding and ordered one of his demons to slaughter the entire family inside.

Then, without hesitation, he slipped out of the city under cover of night, traveling all the way to Mount Fuji—to the forest that would one day be known as the "Suicide Forest."

It was recently declared a restricted zone by the government. No one was allowed in.

Perfect.

After carefully scouting the area, Muzan found a well-hidden cave and started digging.

Then, slowly and meticulously, he buried himself alive.

A smug smile crept across his face.

"Hehehehe…"

"Humans will never outlive me. Not in a million years."

"That so-called strongest swordsman before, Yoriichi? I just buried myself for sixty years and waited him out. And it worked!"

"This time? I'll wait you out for 150 years! I don't care how powerful you are. You're human. That means you'll die eventually."

"You can't beat me. No human ever could."

"I don't care if you turn into a demon by eating other demons. Go ahead—eat them all. What happens when there's no one left to eat? You'll die anyway."

"I've got time. I can wait 150 years."

As he completely sealed himself under the earth, Muzan added one final thought.

"...Actually, make that 200. Just to be safe."

And with that, the self-proclaimed Demon King drifted off to sleep, buried underground.

It wasn't exactly comfortable, but the nap? Oh, it was the best sleep he'd had in centuries.

— — —

The Infinity Castle

Deep underground, far beneath a nameless city, lay the chaotic, labyrinthine space known as the Infinity Castle—controlled by the Biwa-playing demon Nakime.

With no sky, no ground, and no resources, only demons could survive in a place like this. And normally, this is where Muzan would hide out. He even kept a treasure stash here.

But ever since Nakime had… let's say switched sides, Muzan hadn't dared return.

Not a single demon in Japan knew that Muzan had sealed himself in a forest somewhere, hoping to outlive another threat. No one—except maybe Ryo—had the slightest clue.

After all, Ryo could see Muzan for what he really was.

Just a toddler who happened to get his hands on godlike power.

At least, that's how Ryo saw it. And really—was he wrong?

Living for over a thousand years and still accomplishing nothing?

Searching for the Blue Spider Lily for a millennium and still not finding it?

Trying to overcome sunlight but never bothering to study it seriously until the final three years—only to go after Tanjiro's family at the last minute in a desperate bid to make sun-immune demons?

And that, ironically, is what killed him.

Honestly, Ryo was stunned.

A pig that lived for a thousand years in Little Garden would probably evolve and reach a five-digit rank just from having lived that long.

But Muzan? He wasn't even on pig level.

Demon King? Please.

Even Shuten-douji would laugh herself to death.

Ryo was pretty sure he would make a better Demon King. At least he'd try to research something.

And comparing Muzan to other final bosses Ryo had met? It was just sad.

Aizen would literally die laughing.

Now, after invading the Infinity Castle thanks to Nakime, Ryo stood calmly beside a very tense Kanae Kocho.

"Relax," Ryo said with a warm smile. "Just treat this place like a hotel."

"Wait—Muzan might still be in here!" Kanae said, gripping her sword tightly. "Lord Ubuyashiki said this was Muzan's base! What if he left a bunch of demons behind? Even if you're strong, Ryo, being surrounded would still be dangerous, right?"

Nakime blinked, thoroughly confused.

What was with this Demon Slayer girl?

Wasn't Muzan-sama right here beside her?

Why was she talking like he wasn't?

And what's this about Ryo getting hurt? What demon would even dare try that?

Ryo, meanwhile, simply raised a finger to his lips and gave Nakime a playful "shh."

Nakime froze in realization.

Ah… Muzan-sama was toying with the Demon Slayers again.

What a prankster.

A new style of play, maybe? He used to just kill them on sight. What's with the character development?

Wait—no. Stop thinking. Muzan-sama can read minds at this range. Calm down. Don't let him know you're confused.

Meanwhile, Ryo—blissfully unaware of Nakime's spiraling thoughts—turned to Kanae with a chuckle.

"Normally, the only ones in the Infinity Castle are Muzan and Nakime."

"Wait, seriously?" Kanae's eyes widened. "A demon city… with no demons?"

Was that supposed to be a joke?

But Ryo leaned in closer, smiling softly as he whispered in her ear.

"I told you, didn't I? I'd show you firsthand whether demons and humans could coexist."

He pulled back, voice low but firm.

"This place belongs to demons. Soon, you'll meet all of the Twelve Kizuki. And then…"

"You can decide for yourself."

Before she could say anything else, Ryo turned to Nakime and gave a command.

"Nakime, summon the entire Twelve Kizuki."

"Yes, sir."

Nakime struck her biwa, activating her Blood Demon Art.

In the blink of an eye, the remaining Twelve Kizuki—except for Rui, of course—were teleported into the Infinity Castle, each one shocked and confused by what was happening.

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