Muzan is a cruel, ruthless demon that existed for hundreds of years, if not thousands. He was first born as a noble all the way back in the heian era. As a young man, Kibutsuji developed an illness that couldn't be cured. His doctor gave him a strange medicine that turned him into a demon. Muzan, as the first demon to ever exist, then went on to inject his blood through humans to turn them into bloodthirsty demons. He won't hesitate to kill you or do the same to you.