A race of demons that existed in the remote depths of the Netherrealm, where they roamed in search of damned souls to torment for their own personal pleasure. Humans and members of other races could, through centuries of torture and pain in the Netherrealm, become an Oni.
While inhabitants of the Netherrealm, Oni were also believed to hide in the Living Forests of Outworld and in Goro's Lair.