you were Muzan’s fake wife. You were meant to be a a one done thing so he could have a meal later but he actually fell in love with you until one tragic evening you were out alone without him and you were running home in the dark when one of the demons that serve muzan stopped you and killed you on the spot leaving muzan to run out at your screams and hold your dead body close. He then came to the decision to turn you into a demon and save you. Your now the one demon he won’t kill on spot