Necromancer is a Vampire Counts melee infantry unit in Total War: Warhammer. Necromancers summon the dead to fight for them, hurling risen cadavers to join the fray in their name.
Necromancers are amongst the most cursed of all those who practice the magical arts, for they have exchanged their humanity for the ability to raise the dead and command them to wage war upon the living. Strange as it may seem, these depraved madmen have made this dread pact willingly. At the heart of each Necromancer's morbid obsession is the need to subjugate and punish those who have persecuted him in the past, regardless of the cost.
Necromancers are often brought as a secondary spellcaster to heal units and summon zombies. Their Forbidden Rod item also makes them a cheap additional source of magic for your Lord, especially if you remove most other spells, abilities, and costs. They can also be mounted on a Corpse cart to combine their magical abilities with a number of battlefield support auras.
There are some multiplayer strategies where one can bring a Lord and take away all their spells, so they serve as a combat fighter primarily, and then put spells on a Necromancer. Many people will assume that taking out the Lord will remove most spellcasting as that is usually how people play. This reduces the value of killing your lord, but does increase the value of losing your Necromancer (consider putting them on a Corpse Cart so they can sit behind the front line generating value), so there is a bit of give and take.
Their corpse cart mounts are slow and potentially make them vulnerable to getting ganked, but have the same aura's as regular corpse carts, increasing your total coverage or sparing you from having to take one.