For Polo, Prester John (or Un-khan) was a powerful prince who ruled over the Tartars (Mongols), but was overthrown, in a battle Polo himself describes, by the Mongol leader “Chingis-khan.” 

Background on Polo’s expedition. 

See Polo’s route.

Silverberg excerpts the three mentions of Prester John in the Travels:

More on the travels and their veracity: