Saint Jadwiga, queen (but read below) of Poland (Hedwig of Anjou), daughter of Louis I the Great of Hungary, king of Hungary and Poland, acceeded as queen of Poland in 1382/3. She died 1399. Both her mother, El?bieta, and grandmother, El?bieta ?okietkówna, were from Piast dynasty, she was granddaughter of Wladyslaw Lokietek
Jadwiga was technically king of Poland, because the only word for queen in Polish is constructed as wife of king (królowa = król + -owa wife-of morpheme). Nowadays, as morpheme -owa isn't used outside of a few traditional constructs, it's not uncommon to just call her queen.
Jadwiga had to return to Poland, when her sister Katharina died in 1378. Her father Louis, king of Hungary and Poland, died in 1382.
The archbishop of Krakow crowned her as "Jadwiga, king of Poland" - she was probably only woman in history with title of king, not queen. The nobility of Poland urged her to marry Jagiello, Grand Duke of Lithuania.In February 1386 Jagiello took on Christianity and three days later they were married. In March grand duke Jagiello was crowned as "Wladislaw" Jagiello of Poland. She married Jagiello "For the love of god, the good of Poland and to help convert Lithuania to Christianity", according to Rev. Szarnicki 1996.
Lithuania had been christianised before when in 1252/3 Mindowe and his wife were baptized by the bishop of Kulm from Culmer Land in presence of the Master of the Teutonic Knights. When Mindowe was murdered by his nephew, great confusion and complete relaps into paganism had followed.