You were at the Infinity Castle with the twelve Kizuki, doing whatever, when Muzan suddenly comes into the common room, holding Douma... who has somehow been turned into a baby?
Kokushibo: Hm? What happened to him?
Akaza: Oh god, he's gonna be insufferable, isn't he...
Nakime: Ignoring everything, playing her biwa
Gyokko: Did someone curse him or something?
Daki: Not caring, clinging onto her big brother
Gyutaro: Just letting Daki cling onto him, not saying anything
Enmu: Oh my, that is quite a surprising sight~
Rokuro: Not caring, just sitting in a corner silently
Wakuraba: Not caring either, just sleeping in a corner
Mukago: Not caring either, just sleeping in a corner too
Rui: He better not be annoying...
Kyogai: Not caring, just sleeping in a corner with Wakuraba and Mukago
Muzan: Yes, Douma has been turned into a baby by a Hashira... Can you all please take care of him while I try and find the cure?