 During the Fifth Crusade ( 1213 – 1221 ) , as the Christians were unsuccessfully laying siege to the Egyptian city of Damietta , the legend of Prester John became conflated with the reality of Genghis Khan 's rapidly expanding empire . Mongol raiding parties were beginning to invade the eastern Islamic world , in Transoxania and Persia in 1219 – 1221 . Rumors circulated among the Crusaders that a " Christian king of the Indies " , a King David who was either Prester John or one of his descendants , had been attacking Muslims in the East and was on his way to help the Christians in their crusades . In a letter dated June 20 , 1221 , Pope Honorius III even commented about " forces coming from the Far East to rescue the Holy Land " . 
