Its a big dungeon so keep going as far as you can. Eventually you will find a soul gem that shoots lightning at you and an entrance thats gated off. The Glass claw is on a pedestal in that room. Once you get the claw the gate will open. Proceed onward and from there you will run into the lair.
Sometimes the gate closes on you if you wait to long or retreat out of there (for obvious reasons). So I suggest saving once you open the gate.
My character was level 18 here and this was probably my toughest fight yet. Good luck! It was well worth it in the end.