As horrifying as he is mysterious, Muzan Kibutsuji is the ultimate evil in the world of Demon Slayer: Kimetsu no Yaiba. The original demon, and therefore the Demon King, Muzan's reign of terror in Japan lasted for 1,000 years leading into Demon Slayer's story. Nearly immortal in every sense of the word, Muzan wasn't always a demon.
Source：https://www.cbr.com/demon-slayer-muzan-kibutsuji-origin/
