This confused text is a Latin translation of what is thought to be a tract originally written in Arabic by a Christian in Baghdad in 1220-21, but some of the material here was certainly added by its Latin translators. It describes in essence the conquests of Chingis Khan, but instead he is presented as a Christian king named David, great grandson of Prester John, a figure who becomes from this point on a regular feature of the Prester John legend... [A]lthough the text does display some intimate knowledge of the initial movements of the Mongols, the details became so distorted by the time they reached the crusaders that those initial facts became grossly misunderstood.
The text itself attests to how this King David had attacked the King of Persia and conquered several cities along the Asian Steppe, including Bukhara, This King David was a Christian, the bishop reported, and was either the son or the grandson of Prester John—although, Bishop Jacques pointed out, “King David was himself commonly called Prester John.” His kingdom was deep in Asia. His involvement in the affair of the Near East had come about because the Caliph of Baghdad had been threatened with war by a fellow Moslem prince, the Shah of Khwarizm; seeing no other ally at hand, the caliph had requested the Nestorian Catholicos—or Patriarch—of Baghdad to summon King David to his aid, and the king had agreed to defend the caliph against the Khwarizmians” (71).