St. Olga of Kiev
When she was 24 years old, Olga married Igor of Kiev. When their son, Svyatoslav, was just three years old, Igor was killed by the Drevilans, a tribe of early Eastern Slavs. Olga took over the throne for her son, refusing to remarry so that she could preserve the throne for Svyatoslav.
When the Drevilans sent 20 of their best men to try to convince Olga to remarry, she had them buried alive. She feigned acceptance of the Drevilans' proposal and invited some of the top leaders of the tribe to join her for the official acceptance ceremony. When they arrived, Olga burned down their bathhouse with them in it. She later led an attack on the Drevilans, killing over 5,000 men.
As brutal as Olga was, later in life she became one of the first rulers of Rus' to convert to Christianity. She was baptized at Constantinople in 957, taking the name Yelena, after Empress Helena.+
© Wikipedia PD US