Prester John (or Presbyter Johannes) was a legendary figure popular in Europe from the 12th to the 17th century, reportedly a patriarch and king, who ruled a Christian nation lost amidst Muslims and pagans in the Orient, being a descendant of one of the Three Magi.
According to the Junior Woodchuck Guidebook, Marco Polo saw the Lost Crown of Genghis Khan, sometime during his search for the Prester John's land in China. He had claimed to have found the land and told Kublai Khan of it when he reached Cathay. Kublai invaded Tibet and moved into the kingdom.