Muzan Kibutsuji

Muzan Kibutsuji is the primary antagonist in the 2019 anime series Demon Slayer: Kimetsu no Yaiba and the Demon King. He is the progenitor of all demons and the leader of the Twelve Kizuki. A doctor from the Heian Period turned Muzan into a demon to save him from a terminal illness that would have killed him before he was twenty. Muzan is said to be the first demon to ever exist.