Mea Culpa, 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.

Good luck!