Websince muzan originated from the ubuyashiki bloodline, the family has borne the burden of destroying him. In this way, the generations of ubuyashiki's deep hatred. Webdespite his fearsome reputation among slayers and demons alike, muzan had an illness that almost took his life in demon slayer, resulting in his intense fear of. Webmy theory is that at one point point or an another, muzan was forced by his peers to give an heir to the clan (after all the clan need to continue existing) and the child that come out. Webit is revealed that muzan brought the hereditary curse upon the ubuyashiki family that condemns their members to die at young ages of sickness, due to him originally hailing. Webif any demon divulges his last name kibutsuji out loud, the curse activates, which causes their bodies to stop regenerating and allowing muzan's cells inside the demon to take.
