霊鬼 Though some oni can be killed by man-made weapons and others die of natural causes, 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 and 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 as they did before death, though they are often accompanied by an aura or an eerie glow. They are semi-transparent like ghosts, and they often gain additional supernatural powers in addition to the magic they knew in life. Read more: http://yokai.com/reiki/