The One Punch Man (a.k.a Saitama) in Demon Slayer

Night had fallen over the Butterfly Mansion, its usual calmness broken only by the soft chirping of crickets and the distant rustle of trees swaying in the breeze. Inside, the Hashira and their unusual guest — Saitama — were gathered around a low table, enjoying a rare peaceful dinner. Nezuko sat beside Saitama, gnawing playfully on a steamed bun instead of her usual bamboo muzzle.

For once, things felt… normal.

"Man, this food is great," Saitama said, stuffing his face with rice. "Way better than the convenience store stuff back home."

"You eat like you're training for a sumo match," Sanemi grumbled, but even he couldn't hide his slight smile at the bizarre sight of Saitama devouring food faster than Inosuke.

Mitsuri giggled. "It's nice seeing everyone so relaxed! After everything we've been through, we deserve a break, right?"

Rengoku raised his cup. "A toast to peace! And to our bald savior!"

"Don't call me bald savior," Saitama muttered.

Everyone laughed, but just as the warm atmosphere settled, a strange pulse ran through the air. The laughter died instantly. Every Hashira froze, hands unconsciously moving toward their swords.

"Did you feel that?" Giyuu's voice was low and serious.

"Yeah…" Shinobu's brow furrowed. "It felt… ancient. Malevolent."

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Far Away — The Cursed Ground

Deep in the forgotten lands, where not even demons dared to tread, a crack appeared in the earth. Black smoke hissed from the fissure, twisting like serpents into the night sky. The kanji for Sin (罪) glowed a violent crimson, burning brighter with every second.

A hand — slender, unnaturally pale — reached up from the earth, fingers tipped with claws sharper than obsidian. A voice echoed from the void.

"The Slayer of Demons… and the Slayer of Gods… both exist in this era?"

The figure stepped out into the moonlight. Its face was obscured by a mask of countless writhing faces — each one frozen in agony. This was no ordinary demon. This was Sin, a being sealed away long before Muzan ever drew breath.

And now, Sin had returned.

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Butterfly Mansion — Warning from the Crows

The demon crows burst through the windows, cawing frantically, their wings beating like war drums.

"Warning! Warning! Unknown demonic presence detected in the eastern lands! All Demon Slayers are to mobilize immediately!"

Saitama wiped his mouth with his cape. "Guess dinner's over."

"Can you sense it?" Muichiro asked, his usually blank face showing a flicker of concern.

Saitama squinted into the sky. "I don't really sense stuff like you guys, but… something's off. Like static in the air."

Tanjiro stood, gripping his sword. "Then we can't waste time. We have to stop it before it grows stronger."

Rengoku's eyes blazed. "A new enemy means a new challenge! Let's face it head-on!"

Saitama sighed. "I just hope it's not a weakling. It's getting hard to find good fights these days."

Zenitsu was already trembling. "W-wait, shouldn't we at least wait for sunrise? Fighting ancient demons at night is like asking to die!"

"We don't have a choice," Giyuu said. "If we wait, it might be too late."

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The March Begins

The Hashira, Tanjiro's team, and Saitama set off into the night, their silhouettes illuminated by the full moon. They didn't know what awaited them — only that this enemy was unlike any they'd faced before.

Behind them, Nezuko watched from the mansion, her wide eyes reflecting both fear and hope.

For the first time in centuries, something older than Muzan had stirred. And for the first time, the fate of both worlds — Demon Slayer and Saitama's — would hang in the balance.

To Be Continued...

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End of chapter