Necromancy, alternatively known as Nagomancy, Negromancy, Nigromancy and Necyomancy, is the forbidden art of summoning the dead to do one's bidding. This form of sorcery is also used in communicating with spirits as well as simply reanimating the deceased.
In the real-world, spellcasters do this to try and prophecise future events or gain wishes.
In fiction however many Necromancers raise the dead from beyond the grave to act as servants and soldiers in grand schemes of conquest: some of the more extreme examples of Necromancers may also seek to exterminate the living and replace them with the "undead" or the "living dead".
There is also a similar (fictional or otherwise) but different ability to reanimate and influence the dead by by force of the mind alone called "Necrokinesis".