Kazimierz VI the Great

Emperor of Poland, Tsar Protector of the Russians,
King of Lithuania, Duke(Emirate) of Crimea
Kazimierz VI was only 16 when he ascended to the thrown of Poland on the 6th of November, 1530, which happened to be his birth day. The Kingdom he inherited from his father was a large multi-cultural nation stretching from Breslau in the west to Kiev in the east, Danzig in the North to Crimea in the south.
Though his Kingdom was indeed mammoth in its size, Kazimierz had a dream. He envisioned a huge Polish Empire stretching all the way to the far end of Siberia.
To accomplish this, he declared war upon Pskov and its ally Novgorod. The war was over practically before it began. Pskov was annexed within a month and about two months later Novgorod accepted his offer of peace, ceding Ingermanland to Poland and becoming a Polish Vassal.
There were two wars with Sibir and Kazan afterward, both as a result of Sibirian aggression. Each time Poland was victorious, and each time Kazan was forced to cede territory to Poland as payment for their ally’s aggression. Shortly after the second war Poland annexed Novgorod. One year later, on the 14th of November, 1620, Kazimierz was crowned Emperor of Poland and Tsar Protector of the Russians.

The Polish Empire at the end of Kazimierz VI the Great's reign. (Note that Denmark is a Polish Vassel and that Poland owns two provinces off screen to the east.)
On the political front, Kazimierz instituted the Administrative Monarchy and favored a Quality Regular Army.
He also ordered the colonization of a place called “Brazil,” stating that “Poland will not be left out of what is to be gained in the New World.”

Polish Brazil and Trinidad.
Kazimierz VI the Great died in his sleep on the 27th of November 1627, he was 113 years old. For many Polish citizens he the only ruler they had ever known. It was a sad day in the history of Poland.