After the death of Kęstutis, he was a prisoner of Jogaila from 1382 to 1384. Sigismund was baptized in Catholic rite in 1383. In 1384, he escaped captivity and joined his brother Vytautas the Great, who allied himself with the Teutonic Knights. When Vytautas allied with the Teutonic Knights for the second time to fight Skirgaila, Sigismund was a hostage of the Teutonic Knights, together with his family from 1389 to 1398. He became Duke of Navahradak (1390–1440), and Starodub from 1406. He participated in the Battles of Vorskla and the Grunwald. After the death of Vytautas, he supported his cousin Švitrigaila in his fight against Poland, but later was convinced by nobles to join a conspiracy against him.