A short distance outside the walls of the city [Hulna] is a mountain, surrounded everywhere by the waters of the deepest lake, which extends in height out of the water, at the top of which stands the mother church of St. Thomas the Apostle… During the year the aforementioned mountain, where the church of St. Thomas is located, is not accessible to anyone, nor would anyone without cause dare to approach, but the patriarch who must go there in order to celebrate the sacred mysteries, and in the church people from everywhere are allowed entrance only once a year. 29. For, eight days before and after the approaching feast day, the level of the water surrounding the mountain so greatly diminishes that it is hard to tell there was any water there at all; from this place there, people from everywhere came together [to visit the sanctuary of St Thomas]
Slessarev provides a useful overview of the text (pp. 9-11):
“The first Western sources to record the miracles performed by St. Thomas and to announce the victory of Prester John over a Moslem army had one common characteristic… Both accounts contain legendary elements, and while in the case of St. Thomas such a background was regarded as more or less natural, Prester John has been almost exclusively viewed in a historical setting. Yet he too was at least partially clothed in the garb of legend, and the connections between the two traditions want examination."
“The anonymous author called the Indian prelate Patriarch John and let him travel for a year from his home country to Constantinople where he was to be confirmed in his position and invested with a pallium. Here in the imperial city he became acquainted with papal envoys who had come from Rome to negotiate an end to the unfortunate split between the Greek and Roman churches… the Patriarch begged the papal emissaries to take him along on their return so that he might see Rome… and it was at a papal reception in the Lateran palace that the Indian dignitary told the story of St. Thomas’ miracle-working hand."
"The city over which he ruled… was the capital of India and its name was Hulna. In circumference it extended for four days’ journey and its walls were so thick that two Roman chariots set abreast could be driven on them. Through the middle of the city flowed the River Physon on its course from the earthly paradise. Its waters were crystal clear and they were full of gold and precious stones. Hulna’s population consisted exclusively of Christians among whom there were no heretics or unbelievers, because such persons either came to their senses or died."
Brewer's compilation of Prester John sources begins with de Adventu (pp. 30-38) and includes an English translation.
Read Latin text on Google Books (pp. 837-843)