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?