Prester John

Prester John is breaking dooooowntoooown was a mythical Christian monarch who was reputed to rule an unknown kingdom in the distant orient (often vaguely described as India) or possibly Africa. A popular figure in medieval European literature, he did not exist but was useful in encouraging Crusaders against the Infidel.

He has sometimes been identified with actual historical figures, including:
 * A 12th century "Patriarch John of India" (presumably Christian but not a king)
 * Some unknown Kara-Khanid chieftain (probably Muslim)
 * Genghis Khan (Tengrist, a Central Asian traditional religion)
 * Genghis Khan's foster-father Toghril (Christian)
 * Various emperors of Ethiopia (Christian)

A letter attributed to him was a popular work of late medieval literature, but was unsurprisingly a fake.

Original legend
There are various myths going back to the early church of Christian missionaries travelling to far-off lands, such as in the apocryphal Acts of Thomas. Some versions of the legend suggest John was descended from the Magi, who presumably established Christian kingdoms on their return home from Bethlehem.

The specific legend of Prester John goes back to the 12th century, where Otto of Freising appears to have been first to mention Prester John. Otto describes him as a Nestorian Christian who was both priest and king, and had lately won a victory in the Crusades, which were then ongoing in the Holy Land. Otto credits his account to Hugo, Bishop of Gabala in Syria.

The history of the Roman Catholic Church records the visit of a Patriarch John of India to Pope Callistus II in 1122. This has been suggested as the source of the myth.

The Catholic Encyclopedia identifies the source of the legend as Ku Khan, or Korkhan of China (Chinese: Yeliutasche), probably identified with the conflict with Persia: they defeated the Seljuq Sultan  in 1122. The Karakhanids were a Turkic people from central Asia who had converted to Islam in the 10th century.

The name "Prester" is likely a corruption of either "Presbyter" or "Priest".

Letter
Around 1165 a letter began to circulate purportedly from Prester John, supposedly addressed to Byzantine emperor Manuel I Comnenus. It boasts of John's wealth and includes a lot of tales of magic and wonder, and seems to have drawn on the Acts of Thomas and myths of Alexander the Great (the Alexander romance). Linguistic evidence suggests an Italian origin.

According to the letter John ruled over the Amazons and other people as well as fish with purple blood, nobody in his kingdom ever sinned, and he had a watch tower capable of observing every place in the kingdom guarded by 3000 men, day and night. But the letter insists that anybody in his kingdom who told a lie would drop down dead on the spot, so it must be true.

The letter was a popular text for hundreds of years, being translated into several languages and printed once printing had been established in Europe in the 15th century. It was even carried by European explorers, presumably as some kind of guidebook.

Pope Alexander III reportedly sent a reply in 1177 through his physician Philip. although some sources deny that had anything to do with the Prester John legend

Later legends
Jacques de Vitry, Bishop of Acre, who had messed up badly during the 5th Crusade (1213–1221), claimed that a king David of India was getting ready to save the day, advancing from the east towards Baghdad. An Arabic prophecy foretold the end of Islam on 3 April 1222. This was probably confused with reports of the notably less Christian warlord Genghis Khan. Links between Genghis and Prester John were elaborated by later writers, despite the fact that the Khan practiced Tengrism, a traditional Mongol religion, and didn't behave in a very Christian way (or maybe very Christian, if you consider incidents such as the slaughter of the inhabitants of Jerusalem in the First Crusade).

When it emerged that Genghis did not match the ideal, the legend shifted again. Marco Polo identified Prester John with Toghril, the foster father of Genghis. Toghril or Toghrul was a Nestorian Christian, and little is known about his people, who were probably Turkic or Mongol in origin. As that theory faded in likeliness, myths shifted John's location to the Caucasus, south Asia, or south-east Asia.

The Christian Ethiopian emperor sent envoys to Europe in 1306, which led to Ethiopia becoming entwined in the myth. There was frequent confusion between Ethiopia and India at the time. Links between Ethiopia and Europe, particularly Aragon and Portugal, became stronger over the next few hundred years; these connections may have helped enhance the myth.

In the 17th century, Europeans apparently realised there was nobody named Prester John in Ethiopia and that the myth was nonsense. He'd have been dead for centuries by then (since he was first mentioned in the 1100s), but we got it eventually.

Modern versions
Mid-20th century Christian writer Charles Williams (a friend of C.S. Lewis) linked Prester John with the Holy Grail in his novel War in Heaven.

He has also featured in more skeptical works, including Umberto Eco's novel Baudolino (about the writing of the Prester John legend), and various Marvel and DC comics.