Rumbling

Arriving just in time to spare the Wind Hashira, Gyomei tells Sanemi to close his wounds while he takes on Kokushibo personally. The Stone Hashira begins whirling his flail, and Kokushibo, observing through the Transparent World, is delighted by his sheer physical power, noting it has been three centuries since he faced a slayer this strong. As the flail spins faster, air is dragged toward it, and Gyomei hurls the weapon, narrowly missing. He follows instantly with his axe, forcing Kokushibo to bend away, then drives the flail down with Stone Breathing, Second Form. The demon dodges, but the chain coils around his neck; unable to cut it, he ducks free and loses a lock of hair, which burns where it touches the metal.

Kokushibo realizes the chain, axe, and flail are forged of pure iron, a weapon so refined that his own flesh-made sword would burn against it before ever breaking it, and he notes such sunlight-saturated iron did not exist even at the height of Sengoku swordsmithing. Reasoning that closing the distance still favors him, he lunges at Gyomei's neck, only to miss as the Hashira leaps overhead and hurls his axe, tearing the demon's haori. The chain then wraps Kokushibo's blade and snaps it, and the two collide head-on, Moon Breathing, Second Form, against Stone Breathing, Fourth Form. Afterward Kokushibo reveals his sword always regenerates, making strikes against it pointless, while Gyomei bears a fresh cut across his face. Resolving that he can no longer hold back the technique he meant to save for Muzan, Gyomei tenses and manifests his mark.

Gyomei battles Kokushibo and briefly holds him at bay, since the demon cannot destroy the Stone Hashira's pure-iron weapon.

Kokushibo reveals that his sword reforms each time it breaks, nullifying attacks aimed at the blade. After taking a wound across the face, Gyomei manifests his Demon Slayer Mark to close the gap in power against Upper Rank One.

This Infinity Castle Arc chapter, found in Volume 19, foregrounds the strategic interplay between weapon and regeneration. Gyomei's flail-axe-chain combination, made from sunlight-absorbing iron, is one of the few things that visibly harms Kokushibo, while the demon's self-repairing blade negates conventional sword duels. Gyomei choosing to use his mark now, rather than reserve it for Muzan, signals how serious the threat has become.