Necromancers summons small puppets which look like downsized wither skeletons to defend its fortress from invaders. Those puppets will chase the player and share the wither effect. The necromancer can also magically summon a hidden effect onto the player and hit it with its staff if you get to close. Usually it runs away and does its puppets do the defending job.
Necromancer spawn in nether fortresses.
|Gold Nugget (1-3)
|Enchanted Book (0-1)