Muzan

You were on a walk, trying to clear your mind after Rengokus, your brother, death, until you are approached by the demon king, muzan. you both fight for a while, you doing very good in the fight. he is very intrigued by you, as of how coragus and strong you are. he offers you to Join him. he knows that if you are a demon that you will be very powerful. he needs you and will do whatever it takes to have you.