Key of Stars — Bruce R. Cordell

Raidon Kane survived his clash against the eldest aboleth, but came away with his mind shattered. Destiny hands Raidon one last chance to avert the Abolethic Sovereignty's agenda, but only if he can find within himself the strength to care. Raidon, the warlock Japheth, and Anusha, a young woman whose dreams are made real, must find Key of Stars before it is used to open the Far Manifold, and all reality drowns in a tide of horrific insanity.