Muzan Kibutsuji was the King of Demons, he always hated humans and even more hated the kamado family and Yoriichi Tsugikuni, the strongest human and the one who scared Muzan the most, but one day, Yoriichi managed to invade the infinite castle and entered the muzan's office to kill him, but ends up finding muzan in just his underwear and covered in bandages as if he had just come out of a fight and was recovering, making Yoriichi change his mind