He [i.e. Hugh] related also that not many years before a certain John, a king and priest who dwells beyond Persia and Armenia in the uttermost East and, with all his people, is a Christian but a Nestorian, made war on the brother kings of Persians and Medes, called Samiardi, and stormed Ekbatana (the seat of their kingdom).

When the aforesaid kings met him with an army composed of Persians, Medes and Assyrians a battle ensued which lasted for three days, since both parties were willing to die rather than turn in flight. Prester John, for so they are accustomed to call him... emerged victorious.

He said that after this victory the aforesaid John moved his army to the aid of the Church in Jerusalem. But that when he had reached the river Tigris and was unable to transport his army across that river by any evidence he turned towards the north... tarried there for several years... [and] was forced to return home.” 

In analyzing the anecdote that arguably sparked the Prester John fever across Europe, Niayesh (p. 157) notes the structural "ambivalence" of Hugh's account, noting that his story was "caught half-way between the pagan past of classical authorities and the present of Christian Crusaders" insofar as Prester John is "made to fight the long extinct nations of the Medes and Assyrians, rather than directly facing contemporary 'Saracens.'

Even the somewhat contemporaneous historical details do not, in actuality, herald a Christian savior of western Europe. Although this rumor spawned the centuries-long belief in an Eastern potentate capable of uniting Christendom, the initial account of an Eastern anti-Islamic leader was later understood to refer to the deeds of the Qara Khitai, a nomadic Chinese tribe descending from Manchuria. Significantly, this battle took place in Samarkand, not Ecbatana, as Hugh reports. 

Nevertheless, despite historical mistranslation and Iohannes's failure to reach even Byzantium, this rumor helped set in motion, for many Europeans, a belated recognition of the world beyond the Tigris.

Brewer edits and translates the relevant passages of the chronicle (pp. 43-45).

This confused text is a Latin translation of what is thought to be a tract originally written in Arabic by a Christian in Baghdad in 1220-21, but some of the material here was certainly added by its Latin translators. It describes in essence the conquests of Chingis Khan, but instead he is presented as a Christian king named David, great grandson of Prester John, a figure who becomes from this point on a regular feature of the Prester John legend... [A]lthough the text does display some intimate knowledge of the initial movements of the Mongols, the details became so distorted by the time they reached the crusaders that those initial facts became grossly misunderstood. 

This King David was a Christian, the bishop reported, and was either the son or the grandson of Prester John—although, Bishop Jacques pointed out, “King David was himself commonly called Prester John.” His kingdom was deep in Asia. His involvement in the affair of the Near East had come about because the Caliph of Baghdad had been threatened with war by a fellow Moslem prince, the Shah of Khwarizm; seeing no other ally at hand, the caliph had requested the Nestorian Catholicos—or Patriarch—of Baghdad to summon King David to his aid, and the king had agreed to defend the caliph against the Khwarizmians” (71).

"Of interest to the discussion of the legend of Prester John is a passage in which Benjamin described a powerful king in the East. According to the narrative, the king’s name was Kofar-al-Turak, and this Asian king successfully destroyed the king of Persia. Benjamin claimed that Kofar-al-Turak’s forces 'slew many of the Persian army, and the king of Persia fled with only a few followers to his own country.' The account seems contemporaneous with the 1141 defeat by the Kara-Khitai of the Kara-Khanids, who were nominally vassals of the Seljuks. The idea that the forces of Islam could be defeated by conquerors from the East – especially if they were fellow Christians – no doubt was welcome news in Europe. The series of twelfth and thirteenth century papal and royal embassies to the Turkic nomads known collectively as the Mongols was in part due to the credence placed in the account of Benjamin of Tudela." 

In 1876 the Russian scholar Philipp Bruun published a work entitled The Migrations of Prester John, in which he challenged the whole notion that Bishop Hugh's story was a distorted version of the exploits of Yeh·lu Ta-shih. According to Bruun, the prototype of Prester John was the general Ivané (John) Orhelian, commander-in-chief of the army of the kingdom of Georgia. This John Orhelian is One of his f country's national heroes, who fought valiantly for many years to drive the Turks from the Caucasus. In 1123-24 he recaptured from the Seljuks a wide strip of territory in eastern Georgia, including the cities of Tiflis and Ani, and his grateful monarch, King David the Restorer, bestowed on him large grants of land in the reconquered region.

Bruun raised the interesting point that Otto of Freising apparently confused the Georgian city of Ani with the old Persian city of Ecbatana. In a passage of Otto's chronicle somewhat earlier than the Prester John anecdote, Otto, in providing some geographical information apparently received from Bishop Hugh, remarked, "The kings of the Persians ... . have themselves established the seat of their kingdom at Ecbatana, which ... in their tongue is called Rani." The defeat of the Seljuks at Ani in 1123 thus begins to seem a more plausible source for Prester John's victory at Ecbatana than does the triumph of Yeh-lü Ta-shih outside Samarkand.  Moreover, John Orbelian was a Christian-- Greek Orthodox, though, and not Nestorian. And, though he was neither a king nor a priest, the Georgian general did conduct himself in regal fashion: he dined on silver dishes, had the privilege of sitting on a couch at royal banquets while the other princes sat merely on cushions, and the Orbelian family held the hereditary right to provide over the cornations of Georgian kings.