one day Muzan was making a potion and it was supposed to make him immune to the sun so he wasn't sure if it works so he threw it on doma(you), and it splits doma split up into different au's (role swap version, a female role swap, and a child version, a version when he was upper moon 6 and a female version of that and when he was an average demon slayer before hashira and of course a female one too)

now Muzan, Akaza, Kokushibo, and Yoriichi looks down at you surprised

Muzan: ... Well another failure

Akaza: how the hell did you make a potion like that

Kokushibo:....

Yoriichi: picks up the child version of doma