Translation: ogre spirit, demon ghost
Habitat: any; usually haunts the area near its body
Appearance: Some oni can be killed by manmade weapons, and others die of natural causes. But they do not always peacefully pass on to the next life. Some still have unfinished business or karma left to burn off, while others die such violent or passionate deaths that the soul becomes disjointed at the moment of death. They remain in the human world as a demon ghost. Reiki, written by combining the characters for spirit and demon, are the ghosts of oni unable to pass on to the afterlife. Reiki appear just as they did before death, but accompanied by an aura or an eerie glow. They are semi-transparent like ghosts, and often gain supernatural powers in addition to the magic they possessed in life.
Behavior: Reiki have only one motivation: revenge. They bring suffering to the person or people they feel are responsible for their death, or to those who stood against them in life. Reiki will either follow a target or attach themselves to a particular area—often their own grave site—and assault any who come near. They can haunt for centuries. Reiki persist until exorcised by a powerful Buddhist priest.
Legends: There are fewer stories about reiki than about oni, but tales tell of powerful spirits even more fearsome than their living counterparts. One of the most well-known reiki legends takes place at Gangō-ji, a temple in Nara Prefecture. A mysterious force was haunting the temple’s bell tower and murdering children every night. The force was so powerful that not even the most devout priests could identify it, let alone exorcise it. In a story reminiscent of the adventures of Hercules, only the son of a god was strong enough to track down and defeat the demon ghost, saving the children of the temple.