Erika flew through the Infinity Castle, following the powerful demonic signal reported by her Kasugai crow, Rose. The castle's interior shifted endlessly around her, corridors folding into one another, gravity twisting without warning, but soon, the scenery changed.
She entered a region unlike the rest.
A vast, open chamber stretched out before her, its ceiling lost to darkness. Gigantic stone pillars rose from the floor like ancient trees, forming a battlefield large enough to swallow armies. And within that space, two figures were already clashing.
One of them was unmistakable.
The Mist Hashira, Muichiro Tokito.
His blade moved like drifting fog, his presence faint yet lethal as he exchanged blows with his opponent. The other swordsman, however, was something entirely different.
He was not human.
The man stood tall and powerfully built, his physique refined to an almost unnatural degree. Spiky black hair with red-tinted tips framed his face, and he wore a purple-and-black hexagonal-patterned nagagi kimono that fluttered subtly with each movement. In his hand was a long katana, its presence alone radiating pressure.
An overwhelming, dignified aura emanated from him, calm, ancient, and absolute.
Then Erika saw his eyes.
Six of them.
Three eyes were aligned vertically on each side of his face. The middle pair bore engraved kanji, Upper Rank, in one eye and with the number one etched clearly into the other eye. The remaining eyes had black pupils, their irises fractured with cracked, moon-like patterns that seemed to watch everything at once.
Red markings, shaped like flames, traced across his face, undeniably similar to a Demon Slayer Mark.
Which made one thing horrifyingly clear.
This demon had once been a Demon Slayer.
Even now, he was calmly clashing blades with Muichiro, matching the Hashira's movements without strain, as if merely testing him.
Erika stopped midair.
She didn't need Rose to confirm it.
This presence, this pressure,
Upper Moon One.
And Muichiro was fighting him alone.
Muichiro moved.
"Mist Breathing, Seventh Form: Obscuring Clouds."
He moved around Kokushibo with irregular movements, his tempo and speed constantly shifting. His footwork was erratic, his presence fading and reappearing, designed entirely to obscure his opponent and disrupt timing.
Kokushibo watched him for a moment, his six eyes following every motion.
He seemed… impressed.
"I should unsheathe my blade as well," he said calmly.
"…or else it would be rude of me."
"Moon Breathing, First Form: Dark Moon, Evening Palace."
Kokushibo drew his katana and performed a single horizontal slash, the blade tracing a crescent through the air. Along its path, numerous chaotic crescent-shaped blades were born, launching outward in a violent wave straight toward Muichiro.
Erika saw it.
At that dismoved, he would lose an arm. Or a leg.
She moved instantly.
"Blood Breathing, Third Form: Crimson Vine Judgment."
All six ribbons aligned at once and struck forward in a single, instantaneous whip-like lash, aimed directly at Kokushibo's neck. This was the third form of Blood Breathing she had created, a technique meant for concentrated precision rather than wide destruction.
Kokushibo reacted immediately.
He shifted his sword and blocked the attack cleanly. The impact echoed as ribbon met blade, and his six eyes turned toward Erika.
"Interesting," he said calmly.
"You are the girl they told us about. Not a human. Not a demon. What are you?"
Erika didn't answer.
She twisted away as the crescent moon-shaped blades tore through the air around her.
"What are those…?" she thought.
"I think they're his Blood Demon Art," Selena said telepathically.
"And the breathing technique he used… that's Moon Breathing," Erika replied. "He's using both his Blood Demon Art and a breathing style, that's soo unfair. "
Selena said. "You can do the same, Erika-chan."
"That's different," Erika thought back. "He has far more experience than me."
As she continued dodging and searching for an opening, a sudden bang rang out.
Erika turned.
Genya had arrived.
He was already firing at Kokushibo with his pump-action double-barrel automatic shotgun. Almost at the same moment, Muichiro, having recovered, rushed back in and rejoined the attack.
The fight escalated instantly.
Kokushibo turned his attention fully toward Genya.
"Moon Breathing, Fifth Form: Moon Spirit Calamitous Eddy."
Without even swinging his katana, Kokushibo created multiple long, curved slashes layered over one another. The air itself twisted as a vortex of crescent moon blades formed, firing outward in every direction.
The crescent blades flooded the space.
Muichiro reacted instantly.
"Mist Breathing, Third Form: Scattering Mist Splash."
He performed a powerful, swift circular slash, his blade cutting through the air and blowing away the incoming crescent moon projectiles in front of him.
At the same time, Erika snapped her ribbons outward, weaving them together to form a dense shield in front of herself, deflecting the blades that came her way.
But Genya wasn't as lucky.
Before he could even react, several crescent moon blades tore through him.
Both of his arms were severed instantly.
Not only that, his torso was sliced clean in half.
Genya collapsed to the ground.
"Genya!" Muichiro screamed.
Erika froze for a split second, shock flashing across her face.
"Don't worry, Erika-chan," Selena said telepathically. "He can survive. As long as his head isn't destroyed, he might survive anything."
Hearing that, Erika forced herself to steady her breathing.
Right at that moment, Kokushibo appeared beside Genya.
"Hm," he said calmly.
"I see. So you have eaten demons, huh."
He looked down at Genya's ruined body.
"And you survived, even though your torso has been cut in half."
Erika and Muichiro immediately resumed their assault, attacking from both sides to draw Kokushibo's attention away from Genya.
Kokushibo blocked their attacks effortlessly.
"Around three hundred years ago," he said lightly while deflecting their blades, "there was a swordsman who ate demons just like you."
He glanced back at Genya.
"He died when I sliced his body."
His blade shifted slightly.
"So perhaps for you… it will be your head."
After cleanly deflecting their combined assault, Kokushibo turned fully toward Genya once more.
"I have no reason," he said coldly,
"to let a fake demon like you live."
Just before Kokushibo could deliver the finishing blow, a violent gale tore through the battlefield.
"Wind Breathing, Fourth Form: Rising Dust Storm!"
A swordsman burst in, raising his blade high before swinging it horizontally. A powerful, circular whirlwind of slashes erupted outward, tearing through everything in its path from below and forcing Kokushibo to retreat several steps back.
Kokushibo steadied himself and looked up.
"Oh?" he said calmly. "The Wind Hashira."
The Wind Hashira, Shinazugawa Sanemi, stood between Kokushibo and Genya, his blade held low, his presence sharp and violent like a storm given form.
"You got that right," Sanemi snarled.
"I'm the wind that'll rip your head off, bitch."
On the ground behind him, Genya weakly spoke.
"Aniki…"
Sanemi didn't turn around.
"I'm serious here," he said harshly.
"You really are one pathetic little brother."
Genya's eyes widened.
"Why do you think I went as far as killing our own mother?" Sanemi continued.
"Because I wanted to protect you."
Genya froze.
In that moment, he finally understood.
Tanjiro had been right.
His brother had never hated him.
But Sanemi wasn't finished.
"You should've gotten goddamn married," he snapped,
"had some goddamn children, and lived until you became a goddamn old geezer."
His voice shook with anger and pain.
"For all the times you couldn't do a thing for Mom,
for all the times we couldn't do a thing for our siblings, you should've made your goddamn wife and kids as happy as they could be."
He clenched his sword tighter.
"Because I would've made damn sure no goddamn demon ever came near you."
Genya's vision blurred.
"…Sorry, Nii-chan," he said weakly.
"I'm… I'm really sorry."
Erika, watching from the side, blinked.
"How many times did he just say goddamn…?" she thought.
"Don't get distracted by these goddamn brothers," Selena shot back telepathically.
Kokushibo observed the scene with faint interest.
"Hm," he murmured.
"Demon Slayer brothers… how nostalgic."
Sanemi's eyes snapped back to him, bloodlust flaring.
"How dare you dice my little brother up," he growled.
"You creepy-eyed bastard."
His stance lowered.
"You're dead. Dead. Unforgivable."
With that, both of them launched forward at the same time.
The clash began.
Kokushibo's gaze remained calm as he spoke.
"Moon Breathing, Fifth Form: Moon Spirit Calamitous Eddy."
Without even swinging his katana, multiple long, curved slashes layered over one another, forming a massive vortex of crescent moon blades. The air itself screamed as the attack expanded outward in every direction.
Sanemi stepped forward without hesitation.
"Wind Breathing, Third Form: Clear Storm Wind Tree!"
A violent whirlwind of slashes erupted around his body, the rotating blades of wind intercepting the incoming crescent moons. The clash shredded the surroundings, slicing through pillars and stone as both techniques collided.
Kokushibo observed the exchange with interest.
"Not bad," he said evenly.
"I see your physical body and techniques are at their peak."
His six eyes narrowed slightly.
"But in front of absolute strength… it is nothing."
Sanemi, Erika, and Muichiro moved in coordination, attacking from multiple angles to keep Kokushibo occupied. Their blades and ribbons crossed paths relentlessly, forcing Kokushibo to defend and counter without pause.
Behind them, Genya struggled, slowly pulling his body back together.
Sanemi laughed through gritted teeth.
"This is so much fun," he said.
"This is a demon worth killing."
He raised his sword upward toward the right, lifting it high above his head.
"Wind Breathing, Second Form: Claws, Purifying Wind!"
Four vertical slashes came crashing down at once, sharp and ferocious, tearing through the air like the claws of a raging beast as they descended toward Kokushibo.
The battle escalated further.