Okay, so the player can technically be afflicted with negative Illusion effects, but Bethesda put in a failsafe to ensure that it doesn't actually do anything even if you are hit by it. 'I know that it is possible to have the hero temporarily put under the control of an A.I., but I don't know if vanilla Skyrim can do that for PC Illusion effects. Illusion mages could be extremely dangerous and could really add a different type of enemy threat than what we currently have. Being a vampire would give you a pretty big advantage in this case, since you'd be immune unless the caster has the Master of the Mind perk.
What would be even more interesting is if the player was actually susceptible to enemies' illusion spells: you get hit with a frenzy spell, for example, and you watch helplessly as your character starts attacking your follower, assuming you're not of high enough level to resist it.
recognizing that an enemy is immune to a fear spell and not casting it) was more than Bethesda wanted to handle, but it's very odd that the dragonborn seems to have a monopoly on illusion magic. Maybe getting the AI to work appropriately for casting illusion spells (e.g. Enemies, followers, and the like never use fear, calm, or frenzy spells in combat.
As far as I know, the only NPCs who ever use illusion magic are vampires, and even for them, it's just that invisibility spell they cast when they're fleeing from combat.
