The use and manipulation of those who are gone from out world, but just because they're gone does not mean they are useless.
Corpses: The most common form of necromancy is the use of dead bodies, and skeletons as servants. Beings of this class are more concerned with the physical world. Though this is considered the stronger of the two branches, it does require dead bodies to use. Either by finding them, or creating your own.
Spirits: This tree uses spirits, and ghosts as your familiars. Although they have no physical presence in this world and can barley effect physical objects they do move unseen by most people. Making them useful spies, and familiars. While not powerful on it's own this tree is very useful for supplementing your other skills.
Souls are a potent source of mana, and also make for powerful enchanting materials. Not only that but at the highest levels a Mage can learn to manipulate their own soul. At the highest levels a mage can become a lich, but putting their soul in a physical object.
