Monsters are invisible "undefinied" and seemingly pop out of other monsters.
I went to fight a spider that was visibly alone. I got attacked by an enemy that was not there who's hitbox didn't appear until after combat initiated and had no name. It did 600-800 damage against my armor and when i took it off killed me for 1682 damage. I suspect an ultimate spider was either invisible or spawned from a glitch.
These glitched mobs cannot be targeted or attacked.
I came back to the scene of the crim to see what killed me: It was not a spider, but in fact an ultimate hell rogalian (spider in combat text)