Elminster in Hell: Forgotten Realms: Elminster, Book 4

Hell hath such fury...

On the world of his birth he's all but a god, but when an ancient evil banishes him to the depths of the Nine Hells, he's just another lost soul. Elminster, Sage of Shadowdale, Chosen of Mystra, faces his most desperate struggle - to survive, to escape, to cling to his very sanity - and all the forces of the inferno are rallied against him.