The world turned inside out.
Endless corridors unfolded at impossible angles. Floors twisted into walls. Walls stretched into ceilings. Ceilings spiraled into infinity. Lanterns floated where gravity had no meaning, their dim glow illuminating an architecture that defied logic itself.
The Infinity Castle.
Every Demon Slayer in the region was swallowed by space, falling through the void like scattered stars. They struggled to orient themselves midair, fighting panic with instinct. Thankfully, all of them had undergone Hashira training, most managed to land safely.
But landing safely was only the beginning.
This place was filled with demons.
At long last, the final battle between demons and Demon Slayers had begun.
Akira was falling as well, clad in his jet-black Eclipse Armor. The added weight pulled him downward faster than the others, his descent heavier, sharper.
Then he heard her voice.
"Akira!"
He turned his head and saw Erika flying toward him, her wings cutting through the warped space of the castle. They were close because they had been near each other when they were dragged inside. Earlier, Akira had briefly landed on a platform to speak with Muzan. Erika had fallen first, while Akira, weighed down by his armor, was now dropping faster and farther.
Erika flew alongside him and asked, "Where should we go now?"
Akira answered calmly, as if they were following a routine plan.
"As planned, we split up now. But don't forget to talk to me telepathically, like last time."
Erika nodded. "Okay. And remember, if you die, I'll kill you."
With that, she flew away.
Akira stared after her.
"Oh, come on… at least say a different line…"
There was no reply.
Erika didn't look back as she disappeared into the impossible corridors of the Infinity Castle.
Akira spotted the floor rushing up toward him and twisted his body midair, landing in a perfect superhero landing. His armor absorbed most of the impact, the remaining force dispersing harmlessly through the strange surface beneath his feet.
He stood up, raised his hand, and then stopped.
"Wait… what should I say?"
"Who cares? Just say something cool and get on with it," Selena said telepathically.
"Ok," Akira replied.
He raised his hand again, struck a pose, and said in a cool voice,
"WAKEY WAKEY."
Ten black portal gates opened across the floor.
From each gate, a Yoriichi Type Null Unit rose like a soldier answering a silent call. Their black metal bodies glistened under the lantern light, and their eyes glowed red as they fully emerged.
Selena paused. "Why that line?"
Akira replied proudly, "Because it's more fun this way."
Yes, it was just an inventory spell.
Yes, it didn't need a chant.
But he wanted to do it.
The Null Units' black metal bodies glistened. Their red eyes ignited.
"Go," Akira commanded. "Destroy the demons. For the Republic."
The units nodded and scattered in different directions, each moving with a single directive:
Eliminate all demons—except the good ones.
"What republic?" Selena asked. "We're a kingdom."
"Sorry, sister," Akira said as he started running. "My allegiance is to the republic to democracy!"
"You are literally the prince."
"Let me live," Akira shouted, already gone.
The war between demons and demon slayers began.
The demons were strong, but the demon slayers who had endured Hashira Training were not weak either. Every one of them carried Akira's gadgets: poison smoke grenades, poison-coated swords, and even regeneration potions he had made recently. They couldn't regrow lost limbs, but they were enough to save lives.
This was the final battle, after all.
Akira had convinced everyone to carry them.
Because of that, the battle went far better than it had in the original story.
Still, not everyone was strong enough.
In one section of the Infinity Castle, a group of demon slayers found themselves facing four large demons. They fought desperately, pushing themselves to the limit just to survive. Yushiro was with them too; he was extremely important to the plan, but now, facing such powerful demons, he hesitated.
"Do I have to use my powers here…?"
Before he could decide, a massive black shadow flashed past.
A demon's neck was severed cleanly. It turned to ash.
A Type Null Unit stood where it fell.
"The Type Null Unit…!" one Demon Slayer gasped.
Without pause, the unit moved again. Six arms, each wielding a Nichirin blade, spun like a storm. In seconds, the remaining demons were reduced to dust.
In moments, all the demons vanished into dust.
The Type Null Unit then turned its gaze toward Yushiro.
Yushiro froze.
"That guy Akira said they won't attack me… right?If they do, I'm finished…"
The Type Null Unit stared at him for a moment.
Then it turned away and moved on to its next target.
Yushiro let out a long sigh of relief.
Just as his shoulders relaxed, he suddenly felt something grab him.
Yushiro turned around.
What he saw made his breath catch.
A figure clad in jet-black armor, purple eyes glowing faintly within the helmet, stood behind him. It was Akira.
Under the shocked gazes of the surrounding demon slayers, Akira casually raised a hand and said,
"Oh, hey guys. Take care."
Before anyone could react, he turned back to Yushiro and added,
"Come with me if you want to live."
Without waiting for an answer, Akira grabbed Yushiro and leapt forward, his feet barely touching the surface as he began running at full speed through the shifting corridors.
"W–Wait…!" Yushiro shouted. "Where are you taking me?!"
Akira didn't slow down.
"Did you forget?" he said calmly. "We need to find the demon controlling this place first, so hang on tight."
He accelerated even more.
"WAIT…!!" Yushiro screamed as the distorted world of the Infinity Castle blurred past them.
On the other side of the Infinity Castle, the Insect Hashira, Kocho Shinobu, walked cautiously toward a structure that resembled a shrine.
"Where am I…?" she thought.
"I smell blood."
She turned her head toward the source.
At that moment, a voice reached her ears.
Shinobu stopped.
A cheerful voice echoed ahead.
"Ah~! What a relief! I was starting to think no one would come."
A figure stepped into view.
A demon with rainbow-colored eyes, pale skin, and an expression so gentle it almost looked kind. In his arms was the body of a young girl, her flesh already being consumed. Around him, numerous bodies of girls lay scattered across the floor.
Upper Rank Two.
He smiled brightly.
"Ah… you came. Oh… a girl."
His eyes lingered on Shinobu.
"You look young and tasty. Later, I'll have to thank Nakime."
Shinobu froze for a brief moment as she took in the scene.
The demon tilted his head slightly and continued in the same pleasant tone.
"Hello, nice to meet you. My name is Doma."
He smiled wider.
"It's a very nice night tonight, isn't it?"
From the pile of girls' bodies scattered around them, one of them moved.
The girl barely lifted herself from the ground, trembling as she struggled to breathe.
"H-help me…"
Doma turned his head toward her, his smile unchanged.
"Shhh," he said gently. "We're talking right now."
He swung his fan casually.
But the girl vanished.
Doma blinked and turned back, only to see Shinobu holding the girl, her arms steady.
"Are you okay?" Shinobu asked softly.
Doma's eyes widened in amusement.
"Whoa~! You're fast," he said brightly. "Are you a Hashira?"
The next instant.
The girl Shinobu had saved was split in two.
Blood spilled across the floor.
Shinobu hadn't been fast enough.
"Ah, don't worry about her," Doma said lightly. "Leave her over there. I'll make sure to gobble her up later. There we go."
He smiled as he adjusted the two fans in his hands.
Seeing him like that, those fans, that carefree smile, that calm, gentle voice, Shinobu felt her chest tighten.
She remembered her sister's final words.
A demon with blood stains on his head.
A gentle, carefree smile.
A calm voice.
A pair of sharp fans.
The demon stood calmly amid the ruined corridor, posture relaxed, one fan hanging loosely at his side.
"I am the founder of the Eternal Paradise Cult," he said.
Shinobu watched him in silence.
"My duty," he continued, "is to make all my worshippers, including myself, happy."
He stepped forward lightly.
"This girl was screaming for help just now."
The demon looked down at her remains.
"So that's why I saved her."
He glanced at the girl lying nearby.
"And now, I'll eat her up without leaving a single speck."
Shinobu thought, "So this is the demon who killed my sister."
Doma went on, his tone serene.
"See, this girl isn't feeling any pain anymore. No suffering at all. There's nothing to be afraid of, you know. Everyone is afraid of dying. That's why I eat them all up. Now they get to live together forever."
He placed a hand over his chest.
"All the feelings of my worshippers. Blood. Flesh."
His eyes softened, almost nostalgic.
"I make sure to accept, rescue, and guide them all to enlightenment."
Shinobu's gaze sharpened.
Anger flickered beneath her smile.
"There's no way you're sane," she said sweetly.
"Are you right in the head? You're actually making me vomit."
For the first time, the demon blinked.
"Eeeeh?"
Then he brightened.
He took a step closer, his voice lowering with concern.
"How come you're being this harsh?" he asked gently,
"Even though we only met just now?"
Shinobu's hand tightened around her sword.
Her voice trembled, just slightly.
"You're the one who killed my sister. Right?"
"Don't you recognize this haori?"
His eyes widened a fraction.
"Ah!"
Realization dawned, followed immediately by delight.
"You mean the Flower Breath girl from before?"
His smile softened again, regretful.
"She was such a sweet and cute girl."
He sighed.
"I remember now, I missed out on eating her because the sun came up…"
Shinobu moved.
The blade flashed.
In a single step, Shinobu drove her sword straight through his neck.
The impact echoed sharply through the corridor.
The demon staggered.
Then.
He smiled.
"Hm?"
His head did not fall.
The wound did not kill him.
"But I really wanted to eat her all…"
His body twisted unnaturally as frost bloomed across the floor, spreading outward in a chilling wave.
Shinobu's eyes widened, not in fear, but in resolve.
She withdrew her blade and leapt back, landing lightly on her feet.
The demon straightened.
Behind him, ice lotus flowers unfurled, one after another, like a frozen garden blooming in silence.
In this place, the long-awaited battle between them finally began.