What if Demon Slayer didn't end like it was supposed to? It seems that the Slayers haven't killed all of the upper moons, but who they were able to kill was Muzan himself. Now with this new freedom, the remaining demons had dispersed. And this is how the new species of witches came to know existed. So with this new intriguing information, Kokushibo thought to retrieve new power. In his request for knowledge of dark magic you ask for in return to be his wife.