Boris Feodorovich Godunov was "de facto" regent of Russia from 1584 to 1598 and then the first non-Rurikid tsar from 1598 to 1605. The end of his reign saw Russia descending into the Time of Troubles. Wikipedia
The definitive Wikipedia entry for Boris Godunov. Wikipedia is the biggest multilingual free-content encyclopedia on the Internet. en.wikipedia.org/wiki/Boris_Godunov