Atronach (Bone colossus) 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. It's in a sideroom near the Skyshard of this dungeon
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.