Talk:Necromancer/@comment-12957876-20131018031159/@comment-23970955-20131018054734

You also have to be in a part of the dungeon with a specific backwall for them to spawn. I think it's a slabbed wall, which also spawns Rusty Armoured Bones, and Skeleton Snipers. Nearly my whole dungeon was slabbed wall. Sometimes you're unlucky and only have tiny sections, reducing the chance of one spawning.