John, Prester (Priest John), a supposed Christian king and priest of a medieval kingdom in the interior of Asia, the locality of which is vague and undefined. In the 11th and 12th centuries the Nestorian missionaries penetrated into Eastern Asia, and made conversions among the Keraeit or Krit Tartars, which, according to the earliest reports, are said to have included the khan or sovereign of the tribe. Ung (or Ungh) Khan, who resided at Karakorum, and to whom the afterwards celebrated Genghis Khan was tributary. This name the Syrian missionaries translated by analogy with their own language. converting Ung into "Jachanan" or "John," and rendering Khan by "priest." In their reports to the Christians of the West, accordingly, their royal convert figured as at once a priest and the sovereign of a rich and magnificent kingdom. Genghis Khan having thrown off his allegiance, a war ensued, which ended in the defeat and death of Ung Khan in 1202; but the tales of his piety and magnificence long survived, and not only furnished the material of numberless medieval legends (which may be read in Assemani's Bibliotheca Orientalis, 3, 2, 484), but supplied the occasion of several of those missionary expeditions from Western Christendom to which we owe almost all our knowledge of medieval Eastern geography.
The reports regarding Ung Khan, carried to Europe by two Armenian legates in 1145 to Eugene III, created a most profound impression; and the letters addressed in his name, but drawn up by the Nestorian missionaries, to the pope, to the kings of France and Portugal, and to the Greek emperor, impressed all with a lively hope of the speedy extension of the Gospel in a region hitherto regarded as hopelessly lost to Christianity. They are printed in Assemani's Bibliotheca Orientalis. The earliest mention of Prester John is in the narrative of the Franciscan father John Carpini, who was sent by pope Innocent IV to the court of Batu Khan of Kiptchak, the grandson of Genghis Khan. Father Carpini supposed that Prester John's kingdom lay still further to the east, but he did not prosecute the search. This was reserved for a member of the same order, father Rubruquis, who was sent as a missionary into Tartary by St. Louis, and, having reached the camp of Batu Khan, was by him sent forward to Karakorum, the seat of the supposed Prester John. He failed, however, of his hope of finding such a personage, the Khagan of Karakorum, Mangu, being still an unbeliever; and his intercourse with the Nestorian missionaries whom he found established there satisfied him that the accounts were grievously exaggerated. His narrative, which is printed in Purchas's Collection, is one of the most interesting among those of medieval travellers. Under the same vague notion of the existence of a Christian prince and a Christian kingdom in the East, the Portuguese sought for traces of Prester John in their newly- acquired Indian territory in the 15th century. A similar notion prevailed as to the Christian kingdom of Abyssinia, which, in the hope of finding Prester John, was visited so late as the reign of John II of Portugal (1481-95) by Pedro Covilham and Alfonzo di Palva, the former of whom married and settled in the country. See Gieseler's Kirchengeschichte, 3, 3, 43; Ritter's Erdkelndle von Asien, 1, 283 sq.; Schmidt, Forschungqen im Gebiete d. alteren Bildungsgesch. d. Mongolen und Tubeter (Petersb. 1824), p. 162.