Riccoldo da Monte di Croce's Itinerarius
From Brewer (p. 281):
This text repeats the familiar story of the Mongols killing Prester John... 'Then the Tartars made three squadrons. One squadron, with the Great Khan, occupied Cathay, a most wide province, all the way to the farthest India, and there they killed Prester John and occupied his kingdom, and the son of the Great Khan took the daughter of Prester John as wife'
