The Fire in His Hands — Glen Cook

Once a mighty kingdom reigned, but now all is chaos. In the vast reaches of the desert, a young heretic escapes certain death and embarks on a mission of madness and glory. He is El Murid - the Disciple - who vows to bring order, prosperity, and righteousness to the desert people of Hammad al Nakir. After four long centuries, El Murid is the savior who is destined to build a new empire from the blood his enemies. But all is not as it seems, and the sinister forces pulling the strings of empire come into the light. Who and what lies behind El Murid's vision of a desert empire? The first book in the A Fortress in Shadow book.