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He mentions, however, a brother of this John called Unc who ruled over the Crit and Merkit (or Kerait and Mekrit, two of the great tribes of Mongolia), whose history he associates with that of Jenghiz Khan.
Unc Khan reappears in Marco Polo, who tells much about him as "a great prince, the same that we call Prester John, him in fact about whose great dominion all the world talks."
This Unc was in fact the prince of the Kerait, called by the Chinese Tuli, and by the Persian historians of the Mongols Toghral, on whom the Kin emperor of north China had conferred the title of "wang" or king, whence his coming to be known as Awang or Ung Khan.
In the narrative of Marco Polo "Unc Can," alias Prester John, is the liege lord of the Tatars, to whom they paid tribute until Jenghiz arose.