Necromancers are casters who specialize in the domain of the dead and negative energy. They do not only have power over life, death, and undeath, but often employ undead minions to do their bidding. Common people think that necromancers are specialized wizards but many necromancers are sorcerers, clerics, or druids as well.
While arcane necromancers specialize in weakening the living with negative energy and fear, divine necromancers use obscure rituals to affect the souls and the flesh of the dead before they pass to the afterlife. True necromancers specialize in both. A combination of arcane and divine power, true necromancers learn to walk both paths to gain complete power over life and death.
As they grow in power, their very presence unhinges life and bolsters undead. They learn divine rituals and arcane spells, but also gain unique powers or spell-like abilities to grant them an even larger array of power.
Sources Libris Mortis, Own Work