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Prester John (also Presbyter John) was a legendary Christian ruler in India, combining the roles of patriarch and king. The legend of Prester John began in 12th century with two reports of visits of an archbishop of India to Constantinople and of a Patriarch of India to Rome at the time of Pope Calixtus II (1119-1124). These visits cannot be confirmed, evidence of both being second hand reports.
Otto of Freisingen in his Chronicon of 1145 reports that in 1144, he had met, in the presence of pope Eugene II in Viterbo, a certain Hugo, bishop of Gabala, who told him that Prester John was a Nestorian Christian, was descended from one of the Three Magi, and had defeated the Mohammedans in a great battle "not many years ago". After this battle, Prester John allegedly set out for Jerusalem to rescue the Holy Land, but the swollen waters of the Tigris compelled him to return to his own country.
What is very definite is a letter, the Letter of Prester John, believed to be a forgery, which was supposedly written to the Byzantine Emperor Manuel I Comnenus (1143-1180) by Prester John, the King of India. This letter, appearing around 1165, which recounted many marvels of richness and magic, captured the imagination of Europeans and circulated in ever more embellished form for centuries and shortly after the invention of printing in printed form, being still current in the popular culture during the period of European exploration. During the Second Crusade there was also hope that Prester John would come to the aid of the holy cities and capture back Palestine from the Muslims.
The reports were so far believed that Pope Alexander III sent a letter to Prester John via his emissary Phillip, his physician, on September 27, 1177. Phillip was never heard of again. Several Asian tribes were identified with Prester John by travellers, but from the 14th century onward his empire was sometimes placed in Africa, and in the 15th and 16th centuries it became considered to be equivalent to the Christian kingdom of Ethiopia. Prester John was often identified as a descendant of the Magi, or a descendant of St. Thomas, who had supposedly founded an early (and therefore more pure) church in India. When the Mongols invaded Palestine in the 13th century, the Christians inhabiting the remnants of the Crusader States also believed Genghis Khan was Prester John, coming to rescue them from the Muslims. Another possible origin for Prester John is Toghrul Khan, a Nestorian Khan defeated by Gengis. The lingering belief in a Nestorian kingdom in the east accounts for several Christian embassies to the Mongols, in particular the one of William of Rubruck, who was sent to the Tartars by Louis IX in 1253.