In the 9th century the Pechenegs began a period of wars against Kievan Rus'. For more than two centuries they had launched raids into the lands of Rus', which sometimes escalated into full-scale wars (like the 920 war on the Pechenegs by Igor of Kiev, reported in the "Primary Chronicle"). The Pecheneg wars against Kievan Rus' caused the Slavs from Walachian territories to gradually migrate north of the Dniestr in the 10th and 11th centuries. Rus'/Pecheneg temporary military alliances also occurred however, as during the Byzantine campaign in 943 led by Igor. In 968 the Pechenegs attacked and besieged Kiev; some joined the Prince of Kiev, Sviatoslav I, in his Byzantine campaign of 970–971, though eventually they ambushed and killed the Kievan prince in 972. According to the "Primary Chronicle", the Pecheneg Khan Kurya made a chalice from Sviatoslav's skull, in accordance with the custom of steppe nomads. The fortunes of the Rus'-Pecheneg confrontation swung during the reign of Vladimir I of Kiev (990–995), who founded the town of Pereyaslav upon the site of his victory over the Pechenegs, followed by the defeat of the Pechenegs during the reign of Yaroslav I the Wise in 1036. Shortly thereafter, other nomadic peoples replaced the weakened Pechenegs in the Pontic steppe: the Cumans and the Torks. According to Mykhailo Hrushevsky ("History of Ukraine-Ruthenia"), after its defeat near Kiev the Pecheneg Horde moved towards the Danube, crossed the river, and disappeared out of the Pontic steppes.