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 whole kingdom to Ethiopia in the 14th century. The Portuguese pursued him down the African coast, and Ethiopian envoys at the Council of Florence in 1441 had to explain they had no idea who he was. The hosts frame the story as a case study in confirmation bias and note that the search for a phantom helped drive real mapmaking and cross-continental contact.

• How a binary Crusader worldview turned a Buddhist emperor into a Christian savior
• The forged letter’s fountain of youth, gem rivers, and dog-headed men engineered to captivate Europe
• Cognitive dissonance at civilizational scale when the Mongols replaced the expected rescuer
• Why the three Indias of medieval geography made Ethiopia a plausible new home for the myth
• Prester John’s afterlife in Shakespeare, Umberto Eco, and Marvel comics