Rui was born with a frail body that left him unable to walk or go outside. Muzan Kibutsuji appeared and "saved" him by turning him into a demon, giving him a powerful body at the cost of consuming humans.
The rest of the team—Inosuke and Zenitsu—are rescued and sent for treatment at the Butterfly Mansion. Kimetsu no Yaiba Episode 21
Shinobu Kocho attempts to kill Nezuko, but Giyu Tomioka intervenes to protect them, violating the code he previously upheld. Rui was born with a frail body that
The arrival of the Hashira shifts the focus to the Demon Slayer Corps rules regarding Tanjiro and Nezuko. Kimetsu no Yaiba Episode 21
The episode ends with a bound Tanjiro waking up at the Demon Slayer Headquarters, surrounded by the elite Hashira. Demon Slayer: Kimetsu no Yaiba Episode 21 Review