Calabria was part of the Byzantine Empire until the 11th century. A Greek monk, St. John Theristus, operated in the Stilaro Valley during the 9th century. His aghiasma ("holy font") became a popular center of local pilgrimage, and here a Byzantine monastery was founded in the 11th century. After the Norman conquest of southern Italy, it developed as one of the most important Basilian monasteries in southern Italy, maintaining its splendour until the 15th century, with a rich library and numerous art treasures.