Prester John was a priest-king who supposedly ruled a utopian Christian empire from Persia to India, carried an emerald scepter, and descended from the Magi. He never existed, yet popes wrote him letters and explorers died looking for him. This episode traces the legend from 1145, when a Syrian bishop told Pope Eugene III that an eastern Christian king had crushed a Muslim army, a garbled account of the Buddhist Kara-Khitai ruler Yelu Dashi's victory at Qatwan, through the forged Letter of Prester John around 1165 and Pope Alexander III's 1177 reply sent with his physician Master Philip, who vanished.

When the Mongols arrived in the 1220s, Crusaders celebrated the coming of King David, Prester John's supposed heir, only to meet Genghis Khan. Rather than abandon the myth, chroniclers demoted him to a minor tribal chief, then relocated the wh ...