Sviatoslav or Svyatoslav (both: svyä´təsləv), d. 972, duke of Kiev (945–72), son of Igor and of St. Olga. His mother acted as regent for him until c.962, when he came of age. During his reign, which was spent in conquests, he created an empire that stretched from the Volga to the Danube. By 965 he had defeated the Volga Bulgars and the Khazars, thus bringing under Kievan control the entire area of the Volga River. Then, as an ally of the Byzantine Empire, which was at war with the Bulgars, Sviatoslav defeated the Bulgars of the Danube (968) and further extended Kievan control in the Balkans. He was forced to give up the Balkan lands (971), however, in a war with the Byzantine emperor John I. On his way back to Kiev, Sviatoslav was slain by the Pechenegs (or Patzinaks).