Will-o'-Wisps in Oblivion. They are immune to normal weapons (meaning you need magic/daedric/silver to do ANYTHING), they phase in and out (giving them a free pass to get in close), their default attack does absurd damage through the heaviest armor (honestly armor doesn't seem to help at all in this case. I've never bothered checking if their attack gets to ignore it altogether), and they have staggering amounts of health. At a point in the game when a player can slay a dozen trolls at once and vanquish an endless line of daedric champions, a melee player is forced to change what works in ever other enemy in the game and simply dance about swinging wildly hoping to escape without having their willpower drained to nothing.
