The Belgariad, book 2: Castle of Wizardry, Enchanters' End Game
David Eddings1995

The quest may be nearing its end, but the danger continues. After discovering a shocking secret about himself he never could have imagined—all in pursuit of the legendary Orb—Garion and his fellow adventurers must escape a crumbling enemy fortress and flee across a vast desert filled with ruthless soldiers whose only aim is to destroy them. But even when the quest is complete, Garion’s destiny is far from fulfilled. For the evil God Torak is about to awaken and seek dominion. Somehow, Garion has to face the God, to kill or be killed. On the outcome of this dread duel rests the future of the world. But how can one man destroy an immortal God?