Chronicon Syriacum
Makhtbhanuth Zabhne (c. 1258-1286)
Bar-Hebraeus's Chronicle, written in Syriac, aspires to narrate world history from Creation until the current day, in two books (concerning secular and sacred history, respectively).
In the Chronicon Syriacum, which concerns civil and political history, Bar-Hebraeus records an occurrence in the early eleventh century that connects to the legend of Prester John. According to the Chronicon the Mongol Keraits of the East Steppe adopted Nestorian Christianity in 1007. This is significant insofar as the early European legends speaking of a powerful Indian prince John cast this figure too as Nestorian and it was not known at that time whether or not Nestorian Christianity had spread that far east.
Bar-Hebraeus then goes on to mention that it was a King David who, as chief of these same Keraits, was defeated in 1202 by Genghis Khan, who was once King John's vassal. The Syriac documentation of this events matches those of western authors, including the narrative of Marco Polo's journey. There, however, King David/Prester John is known as Ong Khan.
From Silverberg:
How are we to account for William’s [of Rubruck] linking of Togrul and Kuchluk the Naiman (‘King or Presbyter John’)? They were in fact not brothers, nor of the same tribe, nor of the same generation, nor could Togril, who died in 1203, possibly have succeeded to the throne of Kuchluk, who outlived him by sixteen years. A clue to the source of his error can be found in the Chronicon Syriacum of the Syrian cleric Gregory Abulfaraj Bar-Hebraeus, who lived from 1226 to 1286: speaking of the conversion of the Keraits to Christianity, he notes that in the time of the Mongol dominion they were ruled by an ‘Ung Khan who is called Malik Yuhanna,’ that is, ‘King John.’
See Brewer (pp. 169-170) for the English translation of the account of Prester John in the Chronicon.
