"This cat… and this supply box—how did they even get here?!"
The former Flame Hashira looked as if his entire understanding of the world had been upended.
He and Tengen Uzui had been guarding this place without rest, and yet this bipedal cat had apparently been lurking nearby for some time—and they hadn't noticed a thing?
The young master was also stunned, eyes fixated on Kairo Retsu. "Such… a massive sword."
Among the Demon Slayer Corps, most Demon Slayers wield Nichirin Blades. Even the Hashira adjust the shape of their swords based on their fighting style… but a weapon as massive and purely oversized as this katana? This was the first time the young master had seen anything like it.
Just the blade alone towered over the young master's height. It looked far less like a weapon for killing demons, and far more like something used to slay some kind of enormous beast.
"What an extravagant and gigantic weapon!" Tengen Uzui, now fully geared, returned to the courtyard. Kneeling on one knee before the young master, he declared, "Young master, for the sake of slaying Muzan, I—"
"I understand. Don't worry," the young master replied. "With Rengoku-san here protecting us, that is more than enough. In order to defeat Muzan, we too must… believe in every possibility."
"May fortune favor you in battle," said the young master solemnly.
"Then follow me!" Kairo Retsu, with his oversized Long Sword strapped to his back, leapt into the courtyard. Tengen Uzui followed in a flash. "How do we get into the Infinity Castle? Without those strange doors pulling people in…"
"Simple! We'll ride a Hunter's exclusive mode of transport!"
Kairo Retsu flashed a dazzling grin and whistled. From above came a resounding screech in reply.
Tengen Uzui looked up, eyes widening in surprise, as a birdlike creature swooped overhead. Kairo's left arm snapped upward with a grappling hook, catching the wyvern mid-flight. His other hand grabbed Uzui, and the two were pulled into the air.
"LET'S GO! I, THE LONG SWORD HERO OF THE NEW WORLD, HAVE ARRIVED IN STYLE!!!!"
Kairo shouted gleefully—just as the entire scenery around them warped and transformed. The two of them, along with the wyvern, suddenly emerged in the disorienting, upside-down expanse of the Infinity Castle.
"So this is the Infinity Castle!!"
The red-orange glow from interwoven lanterns surrounded them. Now fully restored to peak form, Tengen Uzui couldn't help but exclaim, "Since my comrades are all risking their lives—there's no way I can sit back and wait!!"
"Exactly!"
Kairo Retsu swung from the wyvern with one hand. "LET'S BURN THIS CASTLE TO ASHES!! WOOOOOOO!!!!!"
However, his howling celebration didn't last long. The wyvern suddenly tossed both of them off its back.
Tengen Uzui landed gracefully, elegantly absorbing the impact.
Kairo, on the other hand, face-planted into the wooden floor—limbs splayed out in a perfect human imprint.
"No fall damage, baby!"
Kairo sprang to his feet and muttered something Tengen couldn't quite follow. Then Uzui watched in further disbelief as Kairo pulled out a lantern, unleashing a swarm of glowing, firefly-like Scoutflies that spread outward, extending a clear trail ahead.
"You… alright?" Uzui asked, staring at the human-shaped dent in the floor, then back at Kairo, stunned.
This so-called Guildmaster had just smashed into the floor face-first—and yet showed zero signs of injury? Even knowing these helpers had strange, Blood Demon Art-like powers, seeing it firsthand was shocking.
"Totally fine! That's what makes us Hunters! The envy of all your neighbor's Demon Slayers!" Kairo cheerfully rubbed his nose. "But before we take down Muzan, there's someone else we need to kill—someone just as dangerous…"
Kairo stroked his chin. "The elder brother of the Breathing Techniques founder—Kokushibo…"
"…?"
Kokushibo, with all six of his eyes open wide, stared intently at the strange, glowing insects that had suddenly appeared in front of him.
They had manifested from nowhere and now clung to his body, forming a luminous path as though guiding someone.
And the most baffling part—he couldn't read these insects.
When Kokushibo swung his blade, the Scoutflies evaded his strikes in an instant, beyond even his perception.
"Some trick of the Demon Slayers…?" Kokushibo muttered, just as footsteps echoed through the hallway toward him.
Gripping his blade, he turned to face the incoming figure… the youngest of the Hashira—Muichiro Tokito.
"..."
Just laying eyes on Kokushibo caused immense pressure to bear down on Tokito. The markings in Kokushibo's eyes clearly identified him—Upper Rank One, the pinnacle of the Upper Moons.
Muichiro instinctively swallowed and gripped his Nichirin Blade tightly at his waist, but didn't draw it yet.
"Your name," Kokushibo asked calmly, his voice devoid of emotion. Despite the tension, Muichiro replied with his name.
"I see. The Tsugikuni name has already faded into history," Kokushibo said with a hint of melancholy. "Well, it has been centuries after all."
"Tsugikuni…? Who's that?" Muichiro remained on edge.
"That was my name when I was still human—Tsugikuni Michikatsu," Kokushibo pointed to Muichiro. "And you… are the descendant of our Tsugikuni lineage."
Muichiro's face briefly wavered at the words, but he quickly regained his composure and tightened his grip on his sword.
"Impressive. A fine mental constitution," Kokushibo observed as the boy—his descendant—drew his blade and lunged at him.
Thus began another clash between the Demon Slayer Corps and an Upper Rank demon.
"KAW! I am the crow specifically assigned to Guildmaster Kairo!"
While Kairo and Tengen Uzui raced through the castle, a crow landed squarely on Kairo's shoulder mid-run, clutching tight.
The crow had an air of refinement and bore a talisman created by Yushiro's Blood Demon Art upon its forehead. It addressed Kairo formally: "Though you can communicate with your comrades at will, I am here to assist with up-to-date information from inside the Infinity Castle…"
"You're a lifesaver!" Kairo shouted as he grapple-hooked across platforms. "Got any updates on Kokushibo!?"
"Yes! As of now—"
"The Mist Hashira, Wind Hashira, Stone Hashira, and Genya Shinazugawa are currently engaged in battle with Kokushibo!"