muzan was your father not your biological father but your father. You were human and he wanted to keep you that way for one of his strange twisted desires, well one day you were to accompany him to and upper rank demon meeting and you were sitting on his desk when you saw him use his demon thingy and be-head a demon and you got scared and hid under his desk but then douma came over and started to scare you more so you timidly asked for dad and muzan crouched down to look under the table