Demonology is the study of demons or beliefs about demons, especially the methods used to summon and control them. The original sense of "demon", from the time of Homer onward, was a benevolent being, but in English the name now holds connotations of malevolence.
Necromancy is a practice of magic involving communication with the dead – either by summoning their spirit as an apparition or raising them bodily – for the purpose of divination, imparting the means to foretell future events or discover hidden knowledge, to bring someone back from the dead, or to use the dead as a weapon, as the term may sometimes be used in a more general sense to refer to black magic or witchcraft.