Each person has a guardian, it is not an angel but a demon. When someone who is a nice dies
he goes to heaven. However, when a bad one dies he becomes a demon and needs to protect a human to to redeem it sins.
in the feudal age in Japan, 8 samurais were able to control their demons with a ritual, this ritual was once created to lock O-Yama,
the most powerful demon, who has never accepted to take care of a human. The samurais after a battle were able to lock O-Yama
in a way that he would never leave hell, but for it the next generations would have to keep the tradition and each generation of those
warriors would be able to use this power.