Undead Skeleton Elite enemy in the Rulanyil's Fall Dungeon, spawns in a very slow timer, likely dependent to the defeat of nearby enemies, so clear the room and other elites near it while you wait. It's in the same room and right next to 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.