The conclusion of the Swordsmith Village Arc in Demon Slayer Season 3 dropped massive revelations regarding the past life of the Demon King, Muzan Kibutsuji, and the sequence of events that lead to him becoming the first demon. Initially a fragile young man, whose life was marred by a terminal illness that threatened to take his life before he reached the age of 20, Muzan developed an intense fear of death, which he would carry in the centuries he spent as a demon. After being treated by a doctor with an experimental medicine derived from the Blue Spider Lily, Muzan killed the doctor in frustration, only to realize that he had been cured of his ailment, save for his vulnerability to the sun and intense craving for human flesh.