Sviatoslav I of Kiev

Sviatoslav became Prince of Kiev in 945 after his father Igor's death, though his mother Olga ruled as regent until 962. His decade in power was a blitz of expansion: he conquered East Slavic tribes, defeated the Alans, attacked the Volga Bulgars, and brought down two regional powers - Khazaria and the First Bulgarian Empire. By 969 he'd carved out the largest state in Europe and moved his seat to Pereyaslavets on the Danube. He divided his holdings among his three sons in 970, placing Yaropolk and Oleg over Kiev and Drelinia while giving Novgorod to Vladimir, his son by a housekeeper named Malusha. A committed pagan despite his mother's Christian conversion, Sviatoslav died in an ambush in 972 before consolidating his conquests, and the succession he'd arranged collapsed into fratricide - Yaropolk and Oleg both died, leaving Vladimir alone.