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).

"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."