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).

And he said that the river that passes through Cairo, which we call the Nile, they call the Gyon. And he said that it comes from that country, passing in between two mountains, and he says this because one can find it written that it comes from the terrestrial paradise, just like the Tigris and the Euphrates which, saving the grace of those who say it, it is from there that all four [rivers] come. However, he said that the Nile passes between these two mountains and there is only one small river, and part of a great canyon. And near to this passage, Prester John has had two large towers constructed and a large chain from one side to the other, so that no one can see into that cave, because he said that people used to go in and that, after anyone went in, he would never return. The cause of this, he told me, is that once one is inside it, one hears a very sweet song that makes you never want to leave.
...
And he also told me that if it pleased Prester John, he could easily move the river to another course. But he left it be because there are many Christians living on the aforesaid River Nile.