Necromancers are evil sorcerers that appear in The Lord of the Rings.
Necromancers can sumon spells and bring the dead back alive and can turn people into frogs (and many more animals like rats). Once the have control over zombies and the undead, they make them attack their enemies. They comunnicate with the spirits to predict the future.
Necromancy is dark magic and black spells. Its what necromancers practice. Don't get too close to a necromancer or else you'll be a frog!