Duke of Lithuania and shared his power with his brother Kęstutis. Their successful collaboration is celebrated in Lithuanian historiography, and gave rise to a much debated theory that a tradition of co-rule or diarchy in Lithuania was customary and arose as early as 1285. The Grand Duchy experienced its greatest expansion during their reign. While Algirdas was mostly active in the east, Kęstutis occupied himself by managing the Duchy's interactions with the Teutonic Knights, Poland, and other western European entities. In 1350 Algirdas contracted a second marriage with Uliana of Tver; he chose their son Jogaila as the next Grand Duke. In 1385 Jogaila opened a new chapter in the history of Lithuania by converting the country to Christianity and signing a personal union with Poland, becoming King of Poland. This Polish–Lithuanian union, in various forms, survived until the third partition of the Polish–Lithuanian Commonwealth in 1795. Jogaila's branch of the Gediminids is known as the Jagiellon dynasty. Kęstutis, Duke of Trakai, despite exercising considerable autonomy while controlling the western provinces of the duchy, was loyal to Algirdas and acknowledged his superiority. Kęstutis was a devoted pagan and dedicated his life to defending Lithuania from the Teutonic Knights. A popular romantic legend arose about his marriage to the pagan priestess Birutė of Palanga. They had seven or eight children, including Vytautas the Great. After Algirdas' death in 1377, his son Jogaila became the Grand Duke. At first Kęstutis and his son Vytautas acknowledged Jogaila's rule, but after Jogaila signed the controversial Treaty of Dovydiškės with the Teutonic Knights, Kęstutis seized Vilnius and became the Grand Duke in late 1381. In August 1382 he was imprisoned in Kreva and died there. Vytautas continued his fight for supremacy, and the conflicts between the descendants of Algirdas and Kęstutis lasted well into the 15th century. Manvydas was the eldest son of Gediminas and inherited the territories of Kernavė and Slonim from his father. Little is known about him, and he died soon after Gediminas. It is believed that he was killed in the Battle of Strėva in 1348 along with his brother Narimantas. Narimantas was the second son of Gediminas. He was baptized as Gleb and went on to rule Pinsk, Polotsk, and – as his patrimony by invitation of Novgorod's nobles – Ladoga, Oreshek and Korela. He initiated a tradition of Lithuanian mercenary service north of Novgorod on the Swedish border that