I'm pretty sure this quest is the epic-line quest for Lumul-nar. If I'm right, then you don't need to defeat Ergoth, or even get down to her. All you have to do is get to that platform at the bottom and destroy the purplish relic sitting on a stand there. It comes to mind that you might be starting the instance through the instance finder, which I believe would start the regular instance (and won't allow you to destroy the relic) rather than the epic-line slightly adapted version which will allow is to be destroyed. If that's the problem, I suggest you go back to the dwarf that gives you the quest and see if you can start it again from him, or if he'll tell you how to start the epic-version again.
Celordal: 105 RK, Tiluriel: 105 LM, Jaenoviel: leveling captain, Galynna: leveling champion, and a few other toons -- Runic Knights, Brandywine
Other alts -- Riders of the Great Eagles, Brandywine, originally Riddermark