In 1333, Pope John XXII wrote a letter to the Great Khan of the Yuan Dynasty, appointing a new archbishop of Beijing. In addition to the letter to the Great Khan of the Mongol Empire, the Pope wrote several more letters. The recipients of those letters were Öz Beg Khan of the Kipchak Khanate, King Leo of Armenia, Archbishop Jacopo, all Mongolian monarchs and their people, and Rex Corum. Unlike all the other recipients, controversies still remain over the identity of this last person. There were people who considered Rex Corum to be the King of Goryeo. But the term referring to Goryeo during the Mongol Empire period was not Corea but Caoli. In addition, the Goryeo King was not a key Khan of the Mongols. In consideration of the historical context and the journey of the new archbishop, the most likely candidate for Rex Corum is the Chagatai Khan.