Basically the hashira's and upper moons+ muzan react to the au's of the second upper moon Douma, which are not few and also react to the disturbed past of Douma which was used as an experiment and toy sexual... And he ended up running away and ending up in a cursed city called Winamizawa...

(Good Luck!!! this story you are Douma!!! Or you can be yourself you choose I don't care)