Muzan Kibutsuji is the main antagonist of Demon Slayer: Kimetsu no Yaiba. He is the Demon King, the first of his kind, as well as the progenitor of all other Demons in existence.
A millennium ago, Muzan was turned into a Demon while trying to cure his own terminal illness, and his goal since then was to live without the fear of death and gain true immortality. He attempts to accomplish this by getting rid of his weakness of the sunlight which he is currently doing in 2 ways: finding theBlue Spider Lilyor creating more Demons in hopes that one day one would be able to conquer their weakness of the sunlight.
Muzan is also the one responsible for slaughtering the majority of the Kamado family and transformingNezuko Kamadointo a Demon.