Muzan Kibutsuji

A millennium ago during the Heian Era, Muzan was turned into a demon from an experimental treatment for his terminal illness

and his goal ever since then has been to live without the fear of death and become truly eternal.[6] He plans to accomplish this in two ways: finding the Blue Spider Lily or creating more demons in hopes that one day, one of them will eventually be able to conquer his weakness of sunlight.