Necromancy is the act of conjuring the dead for divination. It dates back to Persia, Greece and Rome, and in the Middle Ages was widely practiced by magicians, sorcerers, and witches.
Necromancy is not to be confused with conjuring devils or demons for help. Necromancy is the seeking of the spirits of the dead. The spirits are sought because they, being without physical bodies, are no longer limited by the earthly plane.