The Lion Of Cairo by Scott Oden
It is the middle of the 12th century . . .
On the banks of the Nile, in a city alive with intrigue, Caliph Rashid al-Hasan rules as a figurehead over a crumbling empire. In the shadow of the Grey Mosque, generals vie for power and influence under the scheming eyes of a venal grand vizier. Warring factions use murder and terror to silence their opponents. Egypt bleeds - and the scent draws her enemies in: the swaggering Shirkuh, who serves the Sultan of Damascus, and Amalric, king of Jerusalem, whose greed is insatiable and whose Crusader knights are hungry for a fight.
Yet all is not lost. In a distant land, there lives an old man who holds the power of life and death over the Moslem world. He has decided to help the Caliph and sends his greatest weapon. A single man. An Assassin. The one they call the Emir of the Knife...