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

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The Crumbling Maze

The walls of The Black Labyrinth continued to shatter, crumbling into nothingness as Saitama's punch ripped through the darkness.

The Hashira could feel the air shifting.

The sky, once consumed by the abyss, slowly returned to normal, and for the first time since entering Tenma's domain, they could see the sun.

But something was wrong.

The shadows weren't fading.

Instead—they were gathering.

"Tch… this guy won't go down easily, huh?" Tengen muttered, gripping his swords.

Giyu narrowed his eyes. "He's… condensing his power into a single form."

Tenma, now standing in the center of the battlefield, had changed.

His once monstrous form had shrunk, but the power radiating from his body was overwhelming. His hair, once flowing like liquid shadow, now stood upright, burning like black fire. His eyes—no longer white—glowed like two eclipses.

And then—a second sun appeared in the sky.

No… not the sun.

A massive, swirling black sphere loomed above them, radiating an oppressive force that distorted the air itself.

Shinobu gasped. "That's not just energy… it's a miniature black hole!"

Tenma grinned.

"I had hoped to save this for a more worthy opponent… but since you refuse to die quietly, allow me to show you the true power of darkness."

He raised his hand, and the black sun pulsed.

The ground beneath them shook violently, cracks forming as gravity itself began pulling everything upward. Rocks, trees, even pieces of the ruined labyrinth were being lifted into the air.

Muichiro felt his feet leave the ground. "This… isn't good."

Tengen struggled to stay balanced. "Damn it, if this keeps up, we'll be crushed before we even get a chance to fight!"

Rengoku roared. "Then we fight before that happens!"

"Flame Breathing, Fifth Form: Flame Tiger!"

A massive, flaming tiger charged toward Tenma, its fangs bared, ready to rip him apart—

But before it could reach him, the black sun pulsed again.

The flames vanished.

The entire attack was sucked into the void.

Rengoku's eyes widened. "What—?!"

Tenma smirked. "This is the power of the Black Sun. Any attack that touches it will be erased from existence."

Shinobu clenched her fists. "Then how do we even damage him?"

Giyu stepped forward, unsheathing his sword. "We don't target the sun. We target him."

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The Clash Begins

With a flash, Giyu vanished—

And reappeared right in front of Tenma, his blade already mid-swing.

"Water Breathing, Eighth Form: Waterfall Basin!"

A spiraling wave of water crashed down, aimed directly at Tenma's chest—

But Tenma simply raised a finger.

BOOM!

A shockwave exploded outward, sending Giyu flying backward.

Tenma laughed. "Pathetic."

But before he could finish mocking, Muichiro appeared behind him.

"Mist Breathing, Sixth Form: Lunar Dispersing Mist!"

A dozen phantom slashes rained down on Tenma—each strike seeming to come from a different direction.

For a moment—it seemed to work.

But then—Muichiro's own attack turned against him.

His own slashes bounced off the Black Sun's gravitational field—redirecting straight back at him.

"Tch—!" Muichiro barely dodged as his own blade scraped against his shoulder.

Tenma chuckled. "Did you really think I wouldn't use gravity as a shield?"

"Then let's test how strong that shield is!"

Tengen charged in, dual blades spinning.

"Sound Breathing, Fourth Form: Constant Resounding Slashes!"

His swords became a blur of explosions, striking at Tenma from multiple angles—each slash causing massive detonations.

BOOM! BOOM! BOOM!

Dust and smoke covered the battlefield.

For a moment—there was silence.

And then—a voice came from within the smoke.

"Nice light show," Tenma said casually.

The dust cleared—revealing him standing completely unharmed.

The Hashira gritted their teeth.

Rengoku clenched his fists. "Damn it… what kind of monster is he?"

Obanai, who had been watching silently, finally spoke.

"He's not a demon anymore."

Everyone turned to him.

Obanai's voice was grim. "He's something else entirely. A god of darkness."

---

The One Punch Solution

As the Hashira struggled to strategize, Saitama just stood there.

Watching.

Waiting.

Bored.

"Man… this guy talks too much."

He cracked his neck, rolling his shoulders.

And then, without warning—he jumped.

A single leap—and he was in front of Tenma in an instant.

Tenma's eyes widened. "Wha—?!"

"Serious Series… Serious Punch."

BOOOOOOM!

The air collapsed.

The sky split apart.

And the Black Sun… shattered.

Tenma didn't just get hit—he ceased to exist for a brief moment.

The ground shook as an earth-shattering shockwave tore through the battlefield. The trees, the ruins, even the very fabric of Tenma's domain—were wiped away.

When the dust settled—

Tenma was gone.

Saitama dusted off his gloves. "That took longer than usual."

The Hashira just stared at him.

Tengen was the first to break the silence. "Damn. Remind me to never get on your bad side."

Rengoku laughed. "A splendid display of power!"

Giyu sighed. "I should've expected this outcome."

Shinobu rubbed her temples. "So much buildup… and then he just ends it in one punch."

Muichiro simply nodded. "As expected."

Saitama stretched. "Welp. That was fun. What's next?"

But before anyone could answer—

The sky turned red.

A deep, ominous voice echoed across the battlefield.

"How interesting. It seems you've removed one of my pawns."

Everyone froze.

They knew that voice.

Muzan.

The true ruler of darkness.

The real battle… was only just beginning.

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To Be Continued…