At least to be best of my knowledge, NPCs cannot use Illusion spells against you. Yet they're potentially very useful and powerful. Personally, I don't like having a whole school of magic in my arsenal NPCs don't seem able to deploy. It would be straightforward to implement - the game seizes control of your character for effect duration and essentially turns you into an NPC while you're illusioned. You come round and discover you've decapitated Lydia in a blind frenzy. It makes a change from shouting her off a cliff. I don't think they'd ever do this as people would be too upset about losing control of their actions, albeit for a very short time, but I think it would make for some potentially very challenging opponents who could cast such spells at a high enough level to affect you and if it's a school of magic, not just you should be doing it. What do other people think?

I've seen enemies that used invisibility at least a couple of times though it's exceedingly rare... Either that or it was just a glitch... I do think it would be amusing if enemies could use some of the frenzy spells on you though... The problem is that they don't tend to be useful unless you are at a distance from them and that usually doesn't happen against casters for long... You'd still end up killing them and just end up with a greater satisfaction about the fact that they screwed themselves over from their own spell...

They can use some Illusion spells like Invisibility and I'm sure I've seen one NPC use frenzy on a group of bandits.