it was invaded, with such uncommon valour and success, by a Nestorian priest, whose name was John, that it fell before his victorious arms, and acknowledged this warlike and enterprising presbyter, as its monarch. This was the famous Prester John, whose territory was, for a long time, considered by the Europeans as a second paradise, as the seat of opulence and complete felicity. As he was a presbyter before his elevation to the royal dignity, many continued to call him presbyter John, even when he was seated on the throne but his kingly name was Ungchan (qtd. in Brewer, p. 261)
