From a post entitled: Top 10 Medieval Urban Legends
The legends of Prester John, popular in Europe from the 12th through the 17th centuries, told of a Christian patriarch and king said to rule over a Christian nation lost amidst the Muslims and pagans in the Orient. Written accounts of this kingdom are variegated collections of medieval popular fantasy. Reportedly a descendant of one of the Three Magi, Prester John was said to be a generous ruler and a virtuous man, presiding over a realm full of riches and strange creatures, in which the Patriarch of the Saint Thomas Christians resided. His kingdom contained such marvels as the Gates of Alexander and the Fountain of Youth, and even bordered the Earthly Paradise. Among his treasures was a mirror through which every province could be seen, the fabled original from which derived the “speculum literature” of the late Middle Ages and Renaissance, in which the prince’s realms were surveyed and his duties laid out. Despite the non-existence of Prester John, the medieval belief in the legend affected several hundred years of European and world history, directly and indirectly, by encouraging Europe’s explorers, missionaries, scholars and treasure hunters.

