The Necromancer is a boss mob found in Necromancer keeps and dungeons.
He mostly spawns unarmed skeletal minions, but sometimes rare/ultra skeletons are spawned as well. If you don't defeat the minions quick enough, they will overhelm you. Many rare skeletons tend to give players the Blindness debuff.
As well as spawning minions, the necromancer can attack by shooting lightning as well as sending little colorful orbs, that, when touched, drain a bit of health from the player and bring it to the necromancer through the same process as it was taken.
Necromancers themselves will shoot health stealing darkness orbs, and on defeat they may drop a Cursed Bone. They possess 120 health and do not have a melee attack.
Full armor (8-10 icons) is recommended.
If you're going to attempt a Stronghold Dungeon, the Necromancer is your last enemy. He will wait in front of the last room. The best way to defeat him without getting into trouble is to get him into the water and finish him off in a corner.