If you’ve got Stygian Abyss, there’s a neat little quest you can do prior to gaining access to The Abyss. It’s fairly simple, and the reward is fairly nice – the Necklace of Diligence, with an Intelligence bonus of 5 and Mana Regeneration of 1.
Check out the walk-through here: Naxatilor’s Necklace Quest – “The Arisen”
(Note: spoilers contained within)