False Dmitriy I
Dmitriy I (Russian: Дмитрий, tr. Dmitrii), historically known as Pseudo-Demetrius I, was the Tsar of Russia from 21 July 1605 until his death on 17 May 1606 under the name of Dimitriy Ioannovich (Димитрий Иоаннович). He is sometimes referred to as False Dmitriy I. According to historian Chester L. Dunning, Dmitriy was "the only Tsar ever raised to the throne by means of a military campaign and popular uprisings."
He was the first, and most successful, of three "impostors" (самозванец, samozvanets) who claimed during the Time of Troubles to be the youngest son of Ivan the Terrible, tsarevitch Dmitriy Ivanovich, who had supposedly escaped a 1591 assassination attempt. It is generally believed that the real Dmitriy died in Uglich and that this False Dmitriy's real name was Grigoriy Otrepyev, although this is far from certain.
Background
Dmitriy I entered history circa 1600, when he made an impression on Patriarch Job of Moscow with his learning and assurance. Tsar Boris Godunov, however, ordered him to be seized and examined, whereupon he fled to Prince Constantine Ostrogski at Ostroh, then in the Polish–Lithuanian Commonwealth, and subsequently entered the service of another polonized Ruthenian family, the Wisniowieckis. Princes Adam and Michał Wiśniowiecki found his story to be convincing, as to who he purported to be, and it gave them an opportunity to get involved in the political turmoil that was transpiring in Muscovy.