Necromancy is control over the undead, along with other powers that trifle with mortality. To use necromantic abilities, you must be wielding or wearing a necromancy gem. You can often find these on weapons, and less often on pieces of armor. (If you are in a beast form, necromancy gems in weapons won't work, but ones in armor will.)
Ability to commune telepathically with nearby dead bodies. Higher skill grants higher chances of persuading recalcitrant spirits, as well as higher comprehension rates when learning from the dead.