Mea Culpa, Lord Valthalak
Use the Brazier of Beckoning to summon Lord Valthalak. Dispatch him, and use Lord Valthalak's Amulet on the corpse. Then, return Lord Valthalak's Amulet to the Spirit of Lord Valthalak.
All is now ready, <name>.
When alive, Lord Valthalak used to inhabit what is now The Beast's chamber. Go there and use the Brazier of Beckoning to summon forth Lord Valthalak. It will reincorporate him, so he'll have to be killed again. I'd take care of The Beast first, and be sure take enough friends along to survive the upper portion of Blackrock Spire.
Once you've dispatched Valthalak, use the amulet on his corpse. With his spirit reunified, he'll likely want his amulet back, as well.
How fortunate for you that I know you're not part of the original group that stole this from me.
I'll take back what is mine now, mortal! Give the amulet to me!
Upon completion of this quest you will gain:
This quest starts at Searing Gorge