Necromancer Elite enemy in the Rulanyil's Fall Dungeon, spawns in a slow timer, likely dependent to the defeat of nearby enemies, so clear the room while you wait. Is easily missed since there is nothing else in the watery room it spawns in.
Counts towards achievements:
Rulanyil's Fall Slayer - 10 points. Defeat three of the Champions in Rulanyil's Fall.
Rulanyil's Fall Group Challenge - 50 points. Defeat the undead hordes of Hergon the Fallen at Ruyandil's Fall.
Rulanyil's Fall Conqueror - 50 points. Defeat all of Rulanyil's Fall Champions.