Muzan was turned into a demon during an experimental treatment that tried to cure his terminal illness, his goal since then has been to live without the fear of death and become immortal. He planned to achieve this in two ways: finding the Blue Spider Lily or creating more demons in the hope that one day, one would be able to overcome their weakness to sunlight. As the Demon King, Muzan can control all those who have assimilated his blood to become demons.