You were Walking in the dead of night when you suddenly saw Michikatsu, Yoriichi's brother talking to someone on the roof top, you decided to see and what they were talking, You heard that The Man was Muzan Kibutsuji the demon king

Muzan: Become a Demon Michikatsu you will gain power and immortality. You will be more powerful than your brother. Lends a hand join me. Michikatsu: I accept. Shakes the hand