False Dmitriy III, also called Pseudo-Demetrius III (Russian: Лжедимитрий III), was the last and most enigmatic of three pretenders to the Russian throne who claimed to be the youngest son of Ivan the Terrible, tsarevich Dmitry.
Supposed to have been a deacon called Sidorka, he appeared suddenly, from behind the river Narova, in the Ingrian town of Ivangorod, proclaiming himself the tsarevich Dmitry Ivanovich, on the 28th of March 1611. The Cossacks, ravaging the environs of Moscow, acknowledged him as tsar on the 2nd of March 1612, and under threat of vengeance in case of non-compliance, the gentry of Pskov also kissed the cross to the thief of Pskov, as he was usually nicknamed. On the 18th of May 1612 he fled from Pskov, was seized and delivered up to the authorities at Moscow, and there executed.
Ivan III was the first Muscovite ruler to use the titles of tsar and ''"Ruler of all Rus'"''.
This pretender to the throne, who came to be known as FalseDmitriy I, gained support in Poland and marched to Moscow, gathering followers among the boyars and other elements as he went.
FalseDmitriy I and his Polish garrison were overthrown, and a boyar, Vasiliy Shuyskiy, was proclaimed tsar in 1606.
For most of the 1600s, Sigismund III was occupied with internal problems of his own, like the civil war in the Commonwealth (the rokosz of Zebrzydowski) and the wars with Sweden and in Moldavia.
Dmitriy attracted a number of followers, formed a small army, and, supported by approximately 3500 soldiers of the Commonwealth magnates' private armies and the mercenaries bought by Dmitriy's own cash, rode to Russia in June 1604.
FalseDmitriy tried to flee from the plotters through a window, but broke his leg and was shot.