The average bandit is Nord, so why would he or she use magic? As for magic users themselves, usually you've interrupted them and their only thought is to kill you quickly and be done with it.

I've had one major NPC use paralyze (Alteration) against my follower and one other major NPC. I've also seen a Vigilants of Stendarr use invisibility (Illusion) several times although this was on my first character before the last update.

The spell would look like a fireball with an absorb health effect on it. (So as to get around Resistance to Fire). It would have make restoration spells and healing potions ineffective while giving you an interface with incorrect information upon it, in order to force the player to run away, rather than just spam healing potions and carry on fighting.