Historia Trium Regum (b. 1375)

John of Hildesheim's Historia Trium Regum links Prester John, St. Thomas, and the Three Magi in a single text for the first time. It was originally written in Latin– though no extant copies survive. There exists an early English translation from which Brewer excerpts the relevant Prester John material 

As Hamilton (p. 181, n. 63) notes, Hildesheim "claimed to have based his work on French translations made at Acre of 'caldayce et hebrayce scriptos' brought there from India." Hamilton also mentions that Hildesheim likely consulted the collected writings concerning the Magi housed in Cologne's cathedral. Numerous manuscripts containing the English translation survive, the earliest which date to the first half of the fifteenth century. 

The dating of the original text is difficult, especially because its authorship was ascribed to John of Hildesheim a century after his death by Johannes Trithemius (1462-1516). Hamilton refers to the text as an early thirteenth century text, but it seems safer to date the text as written before the death of John of Hildesheim, which occurred in 1375. 

The Historia Trium Regum provides a cohesive story that links the historical Magi with the current political reality in Europe, which includes the legend of Prester John. Hildesheim based his story on the Gospel accounts of the three kings as well as the apocryphal commentary on Matthew known as the Opus Imperfectum (5th century). 

Some time after returning to the East after visiting the infant Christ, these kings (Melchior, Balthasar, and 'Jasper') are converted to Christianity by the Apostle Thomas, who served as the. When Thomas died, these Magi then selected an heir to serve as spiritual Patriarch of the Indies. They also elect a secular ruler to act as rex et sacerdos, and they call this leader "Priest John," so-called in reverence to John the Evangelist. In the narrative, Prester John, here called "Preter Johan" the secular ruler of India, rules in tandem with a  “Patriarch Thomas":

Than these thre kynges archebysshoppes and other bysshoppes of comyn [common] assent of all the people chose an other man that was dyscrete to be lord and gouerner of all the people in temporalte. And for this cause that yf ony [any] man wolde ryse or tempte agaynst the patryarke Thomas or agaynst that lawe of god yf so were that the patryarke myght not rule hym by the spyrytuall lawe, than sholde this lorde of temporall lawe chastyse hym by his power. So this lorde sholde not be called a kynge or emperour, but he sholde be called Preter Johan. And the cause is this. For the thre kynges were preestes and of theyr possessyons they made hym lorde. For there is no degree so hygh as preesthode is in all the worlde, nor so worthy. Also he is called Preter Johan in worshyp of saynt Johan the euangelyst [the evangelist] that was a preest the moost specyall chosen and loued of god almyghty. Whan all this was done these thre kynges assygned the patryarke Thomas and Preter Johan, that one to be chefe gouernour in spyrytualte, and that other to be chefe lorde in temporalte for euemlore. And so these same lordes and gouemours of Inde ben [are] called unto these dayes.    (qtd. in Brewer, p. 209)

Thus Prester John becomes a title, an idea echoed in Parzival and Younger Titurel, and an idea that anticpates the Prester John as Dalai Lama narrative path.  

The narrative also extrapolates on the Magi legends, which had circulated around Germany since the time of the original Prester John Letter.

It is also notable that Hildesheim refers to Prester John's son, King David, as an enemy to the Mongols. 

For more on the connection between the Prester John and Magi traditions, see Hamilton.

Read an early English translation online.