Oni were the demonic monstrosities that made up the most powerful forces of the Shadowlands. Oni only gained a physical presence in the realm of mortals if spawned by an Oni Lord or by being summoned from Jigoku by someone willing to share their name in exchange for power.
The Dark Lord of the Shadowlands, the human Daigotsu, might possess any oni he could see, at will. The oni's soul was aware of all actions performed while Diagotsu commanded its body. Daigotsu might return to his own body at will, regardless of range, or might shift his soul to another oni within sight.