Ikiryo is a manifestation of the soul of a living person separately from their body. 

Traditionally, if someone holds a sufficient grudge against another person, it is believed that a part or the whole of their soul can temporarily leave their body and appear before the target of their hate in order to curse or otherwise harm them, similar to an evil eye.However, this temporary separation would result in sickness. If the separation became permanent, the person who held the grudge would die. The Ikiryo are said to be able to possess another living person without the originator even being aware of it Buddhist literture describes the Ikiryo as being particularly difficult to exorcise. The spirits are not tied to whomever they possess, and may freely move about bodies.