The Hashira Unite

Episode 63, the eighth and final installment of the fourth season, adapts Chapters 137 through 139. It opens with a long confrontation between the dying Demon Slayer Corps leader Kagaya Ubuyashiki and Muzan, then escalates into the explosive ambush that draws the Hashira together. The finale ends by transporting Tanjiro, the Hashira, and much of the Corps into Muzan's Infinity Castle, setting the stage for the final battle.

Kagaya versus Muzan

Kagaya reveals that the Ubuyashiki line shares Muzan's bloodline and was cursed for it, surviving only by vowing to hunt him. He argues that true eternity lies in human will, pointing to the Corps that has never been wiped out, and deduces that all demons will die when Muzan dies, which enrages the demon into demanding silence.

The explosion

As the Hashira race to the mansion after an emergency summons, Kagaya detonates explosives laced with makibishi, killing himself, Amane, and his daughters and badly wounding Muzan to slow his healing. The blast is revealed as bait to unite the Hashira at one location.

The trap closes

Tamayo immobilizes Muzan with flesh-seed thorns and stabs him with the cure she perfected from the Asakusa demon's blood. Gyomei shatters Muzan's head with his flail, and the gathered Hashira and Tanjiro prepare a combined assault, only for Nakime's biwa to open doors into the Infinity Castle that swallow the Demon Slayers as Muzan vows to kill them all before dawn.

The first eyecatcher recreates the cover of Volume 16 and the second its extra sketch. The anime keeps Kagaya's eyes white rather than the blackened sclera of the manga, slows the mansion explosion into slow motion, and shows Muzan knocked onto his back instead of standing. It also adds scenes of Inosuke, Genya, Murata, Kanao, and Zenitsu being pulled into the castle, and is the only season finale to include a Taisho Secret in the end credits.