The late autumn evening breeze carried a chilling undertone, stirring the withered leaves across the courtyard.
Two days had passed since the Kasugai Crow shattered the silence.
In those two days, the entire Demon Slayer Corps had entered a state of unprecedented high alert.
All Hashira and swordsmen above Kinoe rank had been secretly gathered at the Ubuyashiki Estate and its surrounding core strongholds.
Meanwhile, the number of surveillance eyes marked with the character "Four" on the outskirts continued to grow, like cold, ever-present gazes fixed upon the area.
Everyone understood that Kibutsuji Muzan had already locked onto their location.
That thousand-year-old Demon King could appear at any moment with his Upper Ranks… or use some vast spatial technique to swallow them whole.
The atmosphere was suffocating, like awaiting a final sentence.
And yet, there was still someone who seemed utterly unbothered.
...
In the backyard of the Butterfly Mansion,
Rin lounged lazily against a wooden pillar in the corridor, dressed in a loose black yukata.
In front of him sat a small stove, a few rice cakes grilling over the fire, with a pot of sake warming beside them.
Tanjiro, Zenitsu, and Inosuke—the three stubborn troublemakers—were seated cross-legged in the center of the courtyard in a rather strange posture.
Their eyes were tightly shut, their breathing so faint it was almost imperceptible, as they attempted to grasp the so-called "See-Through World" under Rin's guidance.
"Don't frown like that. The See-Through World can't be forced. The more you try to think about it deliberately, the more your mind will wander."
Rin flipped the rice cakes on the stove and spoke casually.
"Imagine your consciousness as a pool of still water, so calm that even a pebble falling into it creates no ripple. Listen to the wind, listen to the insects… and then listen to the flow of blood within your own body."
"How is that even possible to hear?!"
Zenitsu was the first to break, frantically scratching his blond hair as he rolled around on the ground.
"My head is full of that six-eyed demon and the one with the fan! I'm going to die! I'll definitely get eaten in the final battle!"
"Shut up, you useless coward! This great me already heard it! I heard the rumbling in my stomach! The Boar God is calling me!"
Inosuke leapt to his feet, drawing his dual blades and slashing wildly.
"That's just because you're hungry."
Rin cut through his delusion without mercy.
He glanced at Tanjiro, who was still holding on. Among the three, this kid truly had the highest potential; his breathing was already beginning to align faintly with the natural rhythm.
"Alright, that's enough for today."
Rin placed the grilled rice cakes onto a plate and poured himself a cup of sake.
"Last-minute cramming will only get you this far. Go clean your swords and get some rest early tonight."
"Yes, Rin-san!"
Tanjiro opened his eyes. Though he had yet to fully grasp the See-Through World, his gaze was unwavering.
After sending the three troublemakers away, Rin picked up his cup and took a sip.
Clack, clack, clack.
The light sound of wooden geta echoed along the corridor.
Kocho Shinobu approached, carrying a tray with several bottles of strangely colored concentrated liquids.
She was no longer in her usual attire. Instead, she had already changed into the black uniform of the Demon Slayer Corps, with her butterfly-patterned haori draped over it.
Even her hair was tied up neatly.
She was clearly ready for battle at any moment.
"Have you finished analyzing the chakra samples?"
Rin gestured toward the bottles in her hands.
"Yes."
Kochou Shinobu knelt opposite him and carefully arranged the vials. A sharp, almost obsessive gleam shone in her usually calm eyes.
"I combined the Yang Release chakra you provided with highly concentrated wisteria poison. The result is… astonishing."
She picked up a pale purple vial and held it toward the moonlight.
"Wisteria poison normally destroys demon cells in a one-sided manner. But Yang Release grants vitality. When the two are combined, I created a compound that forces demon cells to undergo explosive overgrowth in an instant."
A faint trace of approval flashed in Rin's eyes.
"Forcing their cells into uncontrolled proliferation?"
"Exactly."
Kocho Shinobu's lips curved into a dangerous smile.
"As long as this is injected into Muzan or the Upper Ranks, their cells will undergo uncontrolled proliferation under the catalysis of Yang Release. When the rate of division exceeds the limits their bodies can sustain… boom."
She made a small gesture of explosion with her hand.
"Their prized regenerative ability will instead become the very thing that accelerates their collapse. And to prevent Muzan from breaking down the toxin, I also incorporated components from the 'human transformation' drug provided by Lady Tamayo."
She set the vial down, her gaze turning icy.
"Aging, humanization, and cellular collapse. That is the triple hell I've prepared for Kibutsuji Muzan."
Rin listened to her explanation of what could only be described as a terrifying biochemical weapon and raised an eyebrow slightly.
Once this butterfly made up her mind, she was more lethal than the deadliest venom.
"Well done. Keep it with you. When the decisive battle begins, find the right moment and administer a proper dose."
Rin handed her a rice cake from the plate.
"Just don't force yourself into close combat with the Upper Ranks. With your abilities, it's better to stay behind me and strike when the opportunity presents itself."
Kocho Shinobu accepted the rice cake, taking a small bite before giving a faint snort of dissatisfaction.
"Stop underestimating me. Even if my strength isn't the highest, I'm the fastest in the entire Corps right now."
"Yes, yes, the Insect Hashira is unparalleled."
Rin replied casually, though his gaze drifted past the courtyard wall toward the main residence of the Ubuyashiki Estate.
The lights there were still burning.
Ever since recovering his health, Ubuyashiki Kagaya had barely rested these past few days.
He was mobilizing every resource the Demon Slayer Corps had accumulated over a thousand years to prepare the final trap.
Not only had he summoned swordsmen from across the country to return and defend, but he had also ordered the placement of specially prepared explosives and wisteria-based toxin smoke bombs beneath the estate and throughout the surrounding forest—enough to level the entire mountain.
"Rin san..."
Kocho Shinobu swallowed her bite and called out softly.
"Do you think… we can really win?"
Even though their strength had risen drastically after a month of training, the shadow that had ruled the night for a thousand years still lingered over them.
Even she, who was usually so composed and sharp-tongued, could not completely suppress the unease in her heart.
Rin did not answer immediately.
He set down his sake cup and looked up at the pale crescent moon in the night sky.
An autumn breeze passed, lifting the hem of his black yukata.
"Whether we can win isn't something you think about. It's something you carve out with your own hands."
Rin turned his head. His pitch-black eyes held no hesitation, only an absolute confidence that naturally put others at ease.
"A thousand years ago, Tsugikuni Yoriichi cut him into eighteen hundred pieces, scaring him into hiding for centuries."
"Today, I can burn that pathetic nest of his in the Infinity Castle to ashes—body and soul. I'll leave him with nothing."
He reached out and casually draped an arm around Kocho Shinobu's shoulders, pulling her into his embrace.
"With me here, the sky won't fall."
Hearing the steady, powerful rhythm of his heartbeat, the lingering unease in Kochou Shinobu's heart finally faded completely.
She leaned quietly against his shoulder and closed her eyes.
"Mm. I believe in you."
…
Late at night, everything was silent.
In a hidden forest several kilometers from the Ubuyashiki Estate,
The last surveillance eye marked with the character "Four" was cut down by a member of the Kakushi.
But it was already too late.
All the gathered information had already flowed back to that eerie city hidden deep underground, like rivers returning to the sea.
Within the Infinity Castle,
Kibutsuji Muzan sat upon a luxurious Western-style sofa.
He was still dressed in a black kimono, his long hair falling over his shoulders, his pale, handsome face completely expressionless.
Yet the air around him warped visibly under the weight of his compressed killing intent.
"Found you."
Muzan's voice echoed through the empty chamber, laced with cruel satisfaction, like a predator finally tearing away its prey's disguise.
"Ubuyashiki Kagaya… you insect hiding in the shadows. You actually gathered all your forces in one place. Do you think that with that lightning-wielding swordsman backing you, you can stand against me?"
Muzan rose slowly, a tyrannical gleam flashing within his plum-red slit pupils.
"How naive."
"Since you've chosen to gather together, I'll grant your wish."
He turned his head toward Nakime, who stood upon the raised platform behind him.
"Nakime."
"I am here."
Nakime held her biwa and bowed respectfully.
"Prepare to activate your spatial technique. Lock onto the coordinates of the Ubuyashiki Estate."
A terrifying smile spread across Muzan's face, his lips curling wide enough to reveal rows of sharp, white fangs.
"Tonight, I will drag everything there—along with that mountain—into the Infinity Castle."
"I will have that arrogant swordsman and those self-righteous Hashira drained of every last drop of blood by endless demons within this maze with no escape."
"And I will personally capture that demon named Nezuko, devour her, and ascend into a true god!"
As Muzan's roar echoed, Nakime raised the plectrum in her hand and struck the strings of her biwa with force.
Ting—!
A piercing note tore through the fabric of space itself.
The entire Infinity Castle began to tremble violently. Countless halls and corridors twisted and rearranged within the void, like a colossal beast opening its blood-red maw, ready to devour the world.
At that moment, the gears of the final battle began to turn with a thunderous roar.
____
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