A lot of Spoilers for the Manga of Demon Slayer. An Au where Muzan did not agree to the medication of this doctor and was just as basically any Demon not himself while being a Demon. After his Death he then gets reborn as human. (I am very aware this doesn't make sense I don't really care it makes me happy so if you don't like the concept I don't see how its my problem.) The Characters all Originally belong to Koyoharu Gotouge.