Boleslaus II of Mazovia, king of Rus'

Bolesław Jerzy II of Mazovia (Polish Bolesław Jerzy II Mazowiecki, also known as "of Galicia", Boleslaw-Yuri II, and Jerzy-Bolesław Trojdenowicz, 1305/1310 – April 7, 1340) was a ruler of the Polish Piast dynasty who reigned in the originally Ruthenian principality of Galicia. After his death started the Galicia–Volhynia Wars over succession of Galicia and Volhynia. Boleslaw was born between 1305 and 1310 to Trojden I of Masovia from the Piast dynasty, Duke of Czersk and Maria, daughter of Yuri I, prince of Galicia. Since his father was still a ruler of the family's Masovian lands, in 1323 Bolesław, renamed Jerzy, became Prince of Galicia. He also received the Duchy of Belz after the childless death of Andrew of Galicia. In 1331 he married daughter of Grand Duke of Lithuania Gediminas and sister of Aldona of Lithuania, wife of Casimir III of Poland. Name of Bolesław's wife is disputed. T. Narbutt wrote that her pagan name was Eufemia and her Christian name was Maria. Oswald Balzer supposed that her Christian name was Eufemia. Kazimierz Jasiński supposed that whole Narbutt's account was fabrication. In a treaty of 1338 Bolesław Jerzy offered Casimir III of Poland succession to the throne of Galicia-Volhynia. Duke Boleslaw was supported by the many German townspeople already living there. In 1324 Boleslaw located the towns of Volodymyr-Volynskyi on Magdeburg law, and Sanok in 1339.[3] He was poisoned in 1340 by orthodox boyars and died without an heir, before his father who continued rule Masovian principality. After Bolesław Jerzy's death the Kingdom of Galicia was gradually annexed by the kingdom of Poland between 1340 and 1366, during the reign of Casimir III of Poland.

http://en.wikipedia.org/wiki/Boles%C5%82aw_Jerzy_II_of_Mazovia

Boleslaw-Yuri II, Prince of Galicia (Polish Bolesław Jerzy II Mazowiecki, also known as Yuri II of Galicia and Bolesław Trojdenowicz, 1308 – April 7, 1340) was a ruler of the Piast dynasty who reigned the originally Ruthenian principality of Galicia (Central Europe).

Boleslaw was born in 1308 to Trojden I of Masovia from Piast dynasty, Duke of Czersk (Masovia) and Maria, daughter of Yuri I, prince of Galicia. Since his father was still a ruler of the family's Masovian lands, in 1323 Boleslaus, renamed Yuri, became Prince of Galicia. In 1331 he married Eufemija, daughter of Grand Duke of Lithuania Gediminas and sister of Aldona of Lithuania, wife of Casimir III of Poland. In a treaty of 1338 Boleslaw-Yuri offered Casimir III of Poland succession to the throne of Galicia-Volhynia. Duke Boleslaw, he was supported by the many German townspeople already living there. In 1324 Boleslaw founded new towns Volodymyr-Volynskyi on Magdeburg law, and Sanok in 1339.[1] He was poisoned in 1340 by orthodox boyars and died without a heir, before his father who continued rule Masovian principality.

After Boleslaus-Yuri's death the kingdom of Galicia was gradually annexed by the kingdom of Poland between 1340 and 1366, during the reign of Casimir III of Poland