Muzan Kibutsuji's dramatic appearance at the Ubuyashiki Mansion is the climax of the fourth season of "Demon Slayer." There, Muzan encounters the bedridden Ubuyashiki, who reveals their shared yet divergent lineage, marked by a thousand-year curse due to Muzan's demonic transformation. The episode portrays the family's devotion to eradicating demons as a form of atonement.

In the background, the Ubuyashiki children, unaware of the impending danger, play innocently—their singing standing in sharp contrast to the tense encounter.

Meanwhile, Shinazugawa sends warnings to the Hashiras, who are making their way to the mansion with Tanjiro. A poignant flashback reveals that Ubuyashiki never deployed the Hashiras for his personal safety, as he considered them too valuable to risk in protecting humanity.

The situation escalates when the mansion—a sacrificial trap planned by Ubuyashiki to capture Muzan—suddenly bursts into flames and explodes. Though Muzan survives the blast, he is bound by a Blood Demon Art and incapacitated by spikes. In this vulnerable state, Tamayo confronts him and injects him with a transformative medicine meant to turn him back into a human as a final act of atonement for her sins. The episode reaches its peak as Himejima appears and beheads Muzan with a decisive strike.

A subsequent flashback reveals that Ubuyashiki had only confided in Himejima about his plan to use himself as bait—a strategy he had foreseen due to his prophetic abilities.

As Muzan begins to regenerate, the Hashiras realize the challenge of facing him in an all-out battle. The finale leads to an even more dangerous scenario as Muzan activates the Infinity Castle, pulling all demon slayers into a labyrinthine structure that sets the stage for the series' epic confrontations.