A gebbeth is a creature in the world of Earthsea. It is a human, or being, that has been consumed by power, and is no longer itself. The first book of the Earthsea series was based around Ged wishing to destroy the Gebbeth that he had accidentally summoned. A gebbeth takes over a body and does not change it physically, but mentally. The being who had once owned the body is not gone, but may or may not be reversed back to the body without the gebbeth. The spell Ged uses to summon a gebbeth was one that he had found while being tutored, and accidentally casted, causing a gebbeth to appear and overtake a man not far by.
Also, gebbeths can be placed upon one purposely, as a way of sending the gebbeth to destroy the person it consumes, or to trick a person by thinking the body it has consumed was the previous owner. In a time prior to the setting of the Earthsea series, the enemy of Morred turns Ekfarran's brother into a gebbeth and then uses it to trick her into travelling into the Jaws of Enlad.