A Reiki is a ghostly type of Oni in Japanese folklore.
They are considered near-godlike monsters who are created by the vengeful souls of Oni who have died and transcended their previous existence, becoming giant, fearsome ghosts with powers far exceeding those they had while they were alive.
Once a Reiki is created, only the most powerful of priests or the gods themselves can hope to vanquish them, and they are fuelled by hatred for the world, even more than regular Oni, since they experienced the taste of death once and have little desire to experience it again.