One note: unintelligent creatures make for very poor spies, as they have no ability to comprehend what they see or relate it back.
Give them some kind of ability that lets their 'master' see through their eyes or something.
Or perhaps what they see is somehow stored in their organs (like their eyes, or a special sac or something) that can be retrived and examined.
Think about it:
1) Make cool undead spider thing.
2) Send it to go scout for you.
3) Ribwalker goes and wanders around for a while, and then comes back.
4) Necromancer: "Tell me my creation, what did you see?"
5) Ribwalker: *continues to mindless wait for more instructions*
6) Necro: "I said speak!"
7) Ribwalker: *makes unintelligable random chittering noises*
8) Necromancer: "DAMN IT!" :)
It says they can do that near the beginning, must have forgotten to elaborate.
Hmm, how about a super advanced undead spy(der), walks on eight spiders, a ribcage holding the victims brains. It takes lots of victims so it's usually not worth it, but it can think on par with mindflayers and maybe has invisibility as a SLA.