Mirror of History
Speculum Historiale (1256-1269)
One of the great encyclopedic works of the Middle Ages, Vincent of Beauvais' Speculum contains a large chunk of the otherwise lost 1246 account of Simon of St. Quentin's and Ascelin of Lombardia's mission to the Mongols in 1245.
From Guzman (footnote 11 on pg 233):
To read the relevant portion of Vincent's Speculum Historiale in an English translation, see Book 32, chapters 2-52."Vincent, copying from Simon of Saint-Quentin, records the popular rumor that Chinggis Khan married the daughter of King David, son of Prester John, and that the mother of Ogodei Khan was thus a Christian. This and the factors mentioned above led to numerous rumors of the Khan’s imminent conversion.”