Long story short: forgotten beasts went through the caverns and exterminated all of the giant cave spiders before I could capture any, crushing my hopes of setting up a silk farm. Later, five necromancers showed up with an undead horde and I captured four of them. I began to wonder if undead spiders could still shot web. Cue series of failed experiments trying to get the necromancers to raise
anything.
This bug report claims that necromancers won't try to raise anything unless they're actually engaged in battle, which I've confirmed in the object testing arena -- they'll happily sit on a pile of corpses without doing their thing unless they see a hostile creature (and even then will usually try to bruise their opponent to death instead of taking control of the legion of dead legendary axedwarves at their feet). I've been trying to prompt this behavior by building windows and fortifications where they can see my dwarves, but...
Pictured: one human and three dwarf necromancers in a cramped room, with nothing but a single fortification between them and one of my rock haulers. They share the room with a giant cave spider chitin, which goes ignored. Nor did they try to raise the giant bat corpse that was there previously. The thing that's really, really odd is that the rock hauler is happily flipping a switch back and forth, completely oblivious to the hostile necromancers right next to him. No job cancellations, no blue exclamation marks. The necromancers are not chained or in cages -- the only things stopping them from escaping are a locked door and hatch. I doubt the dwarf is a vampire, since a couple of other dwarves haven't reacted to the necromancers either.
So I'm wondering, do dwarves not recognize necromancers as hostile entities? Or are they currently ignoring creatures on the other side of fortifications, or creatures that can't path to them?
Maybe if I release some cavies in there they'll enter combat mode. The only other reason I can think of for their lack of action is that the corpses I've offered are too damaged to revive, though it doesn't explain the lack of fear from my dwarves.