Amulet of the Fallen God

Amulet of the Fallen God is a necklace that gives bonuses to stamina, mana regeneration and spell power.

Source
This item is a quest reward from the quest which starts with Caelastrasz in the Temple of Ahn'Qiraj. To complete the quest, you must turn in one to Anachronos. This item drops off C'thun, the last boss of Ahn'Qiraj.