Shortly after the fall of the last Crusader city of Acre, Nestorian monk turned diplomat Rabban Bar Sauma travels under the command of Arghun Khan to Europe in search of a Ilkhan-European alliance against the Muslims. In OTL, it didn't work as the European nations were starting to enter into conflict again in too a bad way to pay heed to a new Crusade. But what if the Tenth Crusade had been called and this alliance was able to take over most of the Muslim world?