After the death of Kęstutis, he was a prisoner of Jogaila from 1382–1384. 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 1389–1398. He became Duke of Navahradak, 1390–1440, and Starodub since 1406. He participated in the Battle of Vorskla and the Battle of Grunwald. After the death of Vytautas, he supported his cousin Švitrigaila in his fight against Poland, but later was convinced by nobles to participate in a conspiracy against him.
On September 1, 1432 Sigismund became the Grand Duke of Lithuania. He signed Union of Grodno with Jogaila and ceded some territories in Volhynia and Podolia to Poland. However, Švitrigaila was still active and had support from many Eastern Orthodox nobles. In 1434, in an attempt to attract support from these nobles, he issued a privilege to nobles of Eastern Orthodox faith, making their rights equal to the rights of noble Roman Catholics. He guaranteed that no noble, regardless of religion, of Grand Duchy of Lithuania can be imprisoned and punished without a court. The privilege was an important development and accelerated formation of a feudal system.
(Lithuanian) Dundulis, Bronius (2004). "Žygimantas I Kęstutaitis". Lietuvos valdovai (XIII-XVIII a.): enciklopedinis žinynas. Ed. Vytautas Spečiūnas. Vilnius: Mokslo ir enciklopedijų leidybos institutas. 94–96. ISBN 5-420-01535-8.
