Complex sentence: Meanwhile, Jogaila had signed the Union of Krewo, promising to Queen Elizabeth's representatives and the Polish lords' envoys to convert to Catholicism, together with his pagan kinsmen and subjects, if Jadwiga married him.
Split into simpler sentences: Meanwhile, Jogaila had signed the "Union of Krewo" with the representatives of Queen Elizabeth and the Polish lords. In the document, Jogaila promised to convert to Catholicism, together with his pagan kinsmen and subjects, if Jadwiga married him.