Olga was the
widow of Russian ruler
Igor; she became
regent for their young son (the later
Sviatoslav I) on Igor's death in
945. She is the first known
female ruler of the Rus, and also the first one to convert to
Christianity (specifically to the
Orthodox Church of Byzantium).
Since her husband had been killed by a conquered tribe who wanted to stop paying tribute to Kievan Russia, Olga went to great lengths to subjugate this tribe (the Derevlians) and avenge her husband for both personal and political reasons. The Derevlians' towns were burned and most of their people killed or enslaved. She also put into place a better method of collecting the tributes of conquered tribes and appointed officials to control financial and judicial matters, generally strengthening Kievan control.
She also visited Byzantium a few years after her conversion, and tried to get close ties established to the Christian church there, despite the fact that very few of the Rus or those they controlled were Christian. When this didn't go to her complete satisfaction, she also sent to Otto I in what's now Germany for a bishop to come and establish Christian churches in her land. (The man who was sent didn't have enough authority for her, but by then her government had closer ties to Byzantium anyway.) Nonetheless, Olga and Igor's son Sviatoslav I did not convert, though he took over from his mother in 964. She retained a large place in government, helping control Kiev while her son was out expanding the territory their government controlled. Olga died in 969. She was later canonized by the Russian Orthodox Church.