Angry spirits. An official and popular belief in angry spirits developed early in Japan (see Tenjin for the most notorious of such spirits, and No plays for legendary examples). Their most common form is as mu-en-botoke 'unconnected hotoke' (ancestors); those who have died but had or have no-one to perform the Buddhist rites to enable them to leave the world. A belief in the power of angry spirits, the need to pacify them with appropriate rituals and the responsibility for filial piety and reverence that this lays upon the descendants is a favourite theme of Japanese religions, including many of the new religious movements. One way of pacifying angry spirits, exemplified in the case of Tenjin, is to promote them by enshrinement as kami.
A Popular Dictionary of Shinto. Brian Bocking.