Spriggans and vampires use invisibility.
I've always wondered how they would implement mind spells (Fear, Fury, Calm) that strike the player.
I'd imagine that Fear would make you think you had much less health than you actually had. Calm would make all enemies invisible to you (and turn the combat music off.) And Fury would make your allies look like frost trolls.
They wouldn't be able to use the mind-affecting spells.
I can only imagine the crying on these Forums when some enemy Illusionist hit you with a Fear/Fury/Calm spell and you lose control of your character for a bit...
We'd all drown in the bitter tears of "Why would you do this to me Bethesda?!?!?!?!? How can you make me lose control of my character?!?!?!?!" *insert screaming infant sounds here*
Yeah... it's probably best that the enemies (and a limited number of them) are restricted to using only Invisibility and even then only for escape.