Necromancy, also called "The Black Arts", is the practice of raising and commanding the dead.
According to the Dunmer, animation or control of a deceased's flesh or spirit is righteous when practiced according to established cult ritual, but illegal and abominable when practiced by secular sorcerers.
Necromancy is not illegal in the Empire, though body and spirit are protected as property, and may not be used without the permission of the owner, and public display of the living dead is widely prohibited. Morrowind is the only province of Tamriel in which necromancy has been banned and is illegal. Mages may purchase corpses and souls for philosophical research, and may do with them as they wish in private. Necromancers and vampires are widely confused in the public mind, and persecuted, but in remote locations, necromancers are free to pursue their studies without legal interference. Necromancy as a whole is still widely debatable throughout Nirn, and different factions and cults have different reasons for why they approve of or reject this dark art. Necromancy also involves animating the dead (as shown by Mannimarco's Staff of Worms) and the creation of Black Soul Gems, capable of imprisoning the souls of sapient beings. The Mages Guild bans the art of necromancy under the rule of Hannibal Traven, which is apparently why many mages left the guild. However, Falcar of Cheydinhal still studies it until he leaves (though without the Guild's knowledge), judging by the Black Soul Gems found in his room. Most necromancers strive to become Liches, but very little have the means and the knowledge of how to do so. There are many books on the subject of necromancy, such as Necromancer's Moon, a book about the Order of the Black Worm, a necromantic cult, and Corpse Preparation, which describes ways of acquiring and preparing corpses for necromantic purposes.
It is unknown whether or not the Mages Guild ever allowed Necromancers back into the Guild before its collapse during the Great War against the Dominion. As of 4E 200, the College of Winterhold allows (to some degree) the practice of Necromancy, as many of the spells in the Conjuration field of magic are spells that reanimate fallen corpses.