 The reigns of Vladimir the Great (980–1015) and his son Yaroslav the Wise (1019–1054) constitute the Golden Age of Kiev, which saw the acceptance of Orthodox Christianity from Byzantium and the creation of the first East Slavic written legal code, the "Russkaya Pravda".In the 11th and 12th centuries, constant incursions by nomadic Turkic tribes, such as the Kipchaks and the Pechenegs, caused a massive migration of Slavic populations to the safer, heavily forested regions of the north, particularly to the area known as Zalesye.The age of feudalism and decentralization was marked by constant in-fighting between members of the Rurik Dynasty that ruled Kievan Rus' collectively.