Sviatoslav I Igorevich (Old East Slavic: С~тославъ / Свѧтославъ Игорєвичь, Sventoslavŭ / Svantoslavŭ Igorevičǐ; Old Norse: Sveinald Ingvarsson) (c. 942 – 26 March 972), also spelled Svyatoslav was a Grand Prince of Kiev famous for his persistent campaigns in the east and south, which precipitated the collapse of two great powers of Eastern Europe, Khazaria and the First Bulgarian Empire. He also conquered numerous East Slavic tribes, defeated the Alans and attacked the Volga Bulgars, and at times was allied with the Pechenegs and Magyars.
His decade-long reign over the Kievan Rus' was marked by rapid expansion into the Volga River valley, the Pontic steppe, and the Balkans. By the end of his short life, Sviatoslav carved out for himself the largest state in Europe, eventually moving his capital in 969 from Kiev (modern-day Ukraine) to Pereyaslavets (identified as the modern village of Nufăru, Romania) on the Danube. In contrast with his mother's conversion to Christianity, Sviatoslav remained a staunch pagan all of his life. Due to his abrupt death in ambush, his conquests, for the most part, were not consolidated into a functioning empire, while his failure to establish a stable succession led to a fratricidal feud among his three sons, resulting in two of them being killed.
Olga of Kiev, who served as regent during her son's youth
The Primary Chronicle records Sviatoslav as the first ruler of the Kievan Rus' with a name of Slavic origin (as opposed to his predecessors, whose names had Old Norse forms). Nevertheless, Sveinald is the Old East Norse cognate with the Slavic form as attested in the Old East Norse patronymic of Sviatoslav's son Vladimir: Valdamarr Sveinaldsson. This patronymic naming convention continues in Icelandic and in East Slavic languages. Even in Rus', it was attested only among the members of the house of Rurik, as were the names of Sviatoslav's immediate successors: Vladimir, Yaroslav, and Mstislav. Some scholars see the name of Sviatoslav, composed of the Slavic roots for "holy" and "glory", as an artificial derivation combining the names of his predecessors Oleg and Rurik (whose names mean "holy" and "glorious" in Old Norse, respectively).