In the
Vampire: The Masquerade roleplaying game,
necromancy is a form of vampiric
magic which deals with the
dead. On the surface it is similar in practice to other forms of vampiric magic, like
Tremere thaumaturgy or
Koldunic sorcery, in that it involves both
spells that can be cast instantaneously and
rituals which require extensive preparation and can take hours to conduct. Necromancy is far more limited than other forms of vampiric magic, however, in that it deals solely with death, dying, and the spirits of the departed (often called "
wraiths"). One tempted to venture a biased opinion might note that Tremere thaumaturgy can imitate Giovanni death magic (case in point: the
Path of Spirit Manipulation) but necromancy can never imitate most Paths of thaumaturgy.
Necromancy as a magical art is commonly thought to have been developed by the Giovanni bloodline of the Cappadocian clan, though there are inconsistencies between 2nd Edition and 3rd Edition Vampire canon which call this into question (these inconsistencies will be addressed at the end of this writing).
The Cappadocians were past masters at dealing with the physical aspects of death, but in their search for knowledge wished to diversify their capabilities and learn how to control the spirits beyond the Shroud. Upon discovering the Giovanni family, a cabal of mortal sorcerors that dealt heavily in death magick, Cappadocius (founder of the Cappadocian clan) decided to Embrace most of the Giovanni and set them to work on developing the necromantic arts for Cainite use.
The Giovanni succeeded handily, and grew ambitious. In the mid-1400s, Augustus Giovanni led a revolt against Cappadocius, which ended in Cappadocius's diablerie and the destruction of the Cappadocian clan, and the subsequent rise of the new Clan Giovanni. For some reason, the Giovanni usurpation of the Cappadocians did not meet the same negative response as the Tremere destruction of Clan Salubri did...
The Giovanni, over the centuries, have refined their necromantic arts into a series of magical paths, each of which deals with a separate branch of practice. Some -- the most common -- deal with communicating with and controlling the spirits of the dead, and interacting with their world. Others deal with the soul, or the dead body. Countless rituals have also been designed. Since wraiths are so very omnipresent in the World of Darkness, they are useful servants indeed.
There are, as I have mentioned above, inconsistencies to the idea that the Giovanni are the first users of necromancy. For instance, the Nagaraja have practiced a discipline known as Nihilistics for millenia; in the 2nd Edition Vampire Storyteller's Handbook Nihilistics is specifically referred to as a discipline, and it is detailed up through the 9th level (necromantic, thaumaturgical and sorcerous paths of magic only go through level 5 in all the World of Darkness games). However, in the 3rd edition storyteller's handbook, Nihilistics is renamed as "Vitreous Path Necromancy" and is given only five levels, with the notation that it is often referred to by the Nagaraja as Nihilistics. Clearly, White Wolf has done some editing here; it's up to the storyteller to determine which version is preferred.
Next, the clan disciplines of the Kiasyd bloodline are given (in the 2nd Edition Guide to the Sabbat) as being Mytherceria, Necromancy and Obtenebration; however, in 3rd Edition, Necromancy is replaced with Dominate. More editing.
It is important to note that in recent nights, the Cappadocians have returned, calling themselves the Harbingers of Skulls now. They have refined their original discipline of control over death processes -- called Mortuus -- into a necromantic path, called the Mortis Path. They have tentatively joined the Sabbat, and hunt their Giovanni usurpers with a vengeance.