sentences:  In 1391, Henry, who was known both for his incredible strength and his diplomatic skills, accepted a mission from the hands of the king of Poland, Władysław Jagiełło.Jagiełło asked him to convince his brother, Vytautas, who at that time opposed Władysław's rise to power, to distance himself from his allies — the Teutonic Knights, enemies of the Kingdom of Poland (and her ally, the Grand Duchy of Lithuania).Henry was successful in convincing Vytautas to abandon the Knights (see Ostrów Agreement), and even more, became friends with Vytautas, who decided to marry him off to his sister, Ryngałła.