From Diablo Wiki
As you encounter Kurast and the overridden Jungles beyond it, Deckard Cain orders you the quest to rekindle Khalim's Will by combining 4 quest items into the Horadric Cube: Khalim's Eye, found in the Spider Cavern, Khalim's Brain, found in the Flayer Dungeon, Khalim's Heart, found in the Kurast Sewers, and Khalim's Flail, found in the Travincal. Once you have 'transmuted' all of these quest items, Khalim's Will appears and Deckard Cain entices you to use it to destroy the Compelling Orb whilst you kill the High Council guarding it. Once you use the Will to destroy the orb, the Will will be unequipped and will no longer appear in your inventory, but fortunately you are able to enter the Durance of Hate and confront Mephisto.
Copyright IncGamers Ltd 2016