Question: When did the cousin of the Grand Duke of Lithuania die?

full_context: Document 1: Kęstutis: Kęstutis (] ; born ca. 1297, died on 3 August or 15 August 1382 in Kreva) was a ruler of medieval Lithuania. He was the Duke of Trakai and governed the Grand Duchy of Lithuania, 1342–82, together with his brother Algirdas (until 1377), and with his nephew Jogaila (until 1381). He ruled over the Lithuanians and Ruthenians.
Document 2: Lithuanian Civil War (1381–84): The Lithuanian Civil War of 1381–1384 was the first struggle for power between the cousins Jogaila, Grand Duke of Lithuania and later King of Poland, and Vytautas the Great. It began after Jogaila signed the Treaty of Dovydiškės with the Teutonic Knights which was aimed against his uncle Kęstutis, father of Vytautas. Kęstutis briefly seized power in the Grand Duchy, but was betrayed by adherents of Jogaila primarily from Vilnius. During negotiations for a truce Kęstutis and Vytautas were arrested and transported to the Kreva Castle. Kęstutis died there a week later but Vytautas managed to escape and then sought an alliance with the Teutonic Knights. Subsequently their joint forces raided Lithuanian lands. Eventually the cousins were reconciled as Jogaila needed internal stability in anticipation of negotiations with the Grand Duchy of Moscow and the Kingdom of Poland regarding the possible Christianization of Lithuania. The war did not settle the power struggle; it continued during the next Lithuanian Civil War (1389–1392) which was resolved by the signing of the Ostrów Agreement. After more than ten years of struggle, Vytautas finally became the Grand Duke of Lithuania and ruled the country for thirty-eight years.
Document 3: Algirdas: Algirdas (Belarusian: Альгерд , Russian: Ольгерд , Polish: "Olgierd" ;  1296  – May 1377) was a ruler of medieval Lithuania. He ruled the Lithuanians and Ruthenians from 1345 to 1377. With the help of his brother Kęstutis (who defended the western border of the Duchy) he created an empire stretching from the present Baltic states to the Black Sea and to within fifty miles of Moscow.
Document 4: Ostrów Agreement: The Ostrów or Astrava Agreement (Lithuanian: "Astravos sutartis" , Belarusian: Востраўскае пагадненне , Polish: "Ugoda w Ostrowie" ) was a treaty between Jogaila (Władysław II Jagiełło), King of Poland and Grand Duke of Lithuania, and his cousin Vytautas the Great, signed on 4 August 1392. The treaty ended the destructive Lithuanian Civil War, launched in 1389 by Vytautas who hoped to gain political power, and concluded the power struggle between the two cousins that erupted in 1380 after Jogaila secretly signed the Treaty of Dovydiškės with the Teutonic Knights. The Ostrów Agreement did not stop attacks from the Teutonic Knights and the territorial dispute over Samogitia continued up to 1422. According to the treaty, Vytautas became the ruler of Lithuania (styled Grand Duke), but he also acknowledged Jogaila's (styled Supreme Duke) rights to Lithuania. The details of the Polish–Lithuanian relationship were clarified in several later treaties, including the Union of Vilnius and Radom in 1401 and Union of Horodło in 1413.
Document 5: Algirdas: Algirdas (Belarusian: Альгерд , Russian: Ольгерд , Polish: "Olgierd" ;  1296  – May 1377) was a ruler of medieval Lithuania. He ruled the Lithuanians and Ruthenians from 1345 to 1377. With the help of his brother Kęstutis (who defended the western border of the Duchy) he created an empire stretching from the present Baltic states to the Black Sea and to within fifty miles of Moscow.
Document 6: Vilikaila: Vilikaila or Viligaila was one of the 5 elder Lithuanian dukes mentioned in the peace treaty with Halych-Volhynia in 1219. He is mentioned as brother of Daujotas, which leads scholars to believe he was the younger or perhaps less influential brother. The brothers are not mentioned in any other sources. Tomas Baranauskas, a modern Lithuanian historian, believes that Vilikaila and Daujotas might be sons of Stekšys, a Lithuanian duke killed in 1214.
Document 7: Lithuanian Crusade: The Lithuanian Crusade was a series of campaigns by the Teutonic Order and the Livonian Order, two crusading military orders, to convert the pagan Grand Duchy of Lithuania into Roman Catholicism. The Livonian Order settled in Riga in 1202 and the Teutonic Order arrived to Culmerland in 1230s. They first conquered other neighboring Baltic tribes – Curonians, Semigallians, Latgalians, Selonians, Old Prussians (see Livonian Crusade and Prussian Crusade). The first raid against the Lithuanians and Samogitians was in 1208 and the Orders played a key role in Lithuanian politics, but they were not a direct and immediate threat until 1280s. By that time the Grand Duchy of Lithuania was already an established state and could offer organized defense. Thus for the next hundred years the Knights organized annual destructive "reise" (raids) into the Samogitian and Lithuanian lands but without great success: border regions in Samogitia and Suvalkija became sparsely inhabited wilderness, but the Order gained very little territory. The Grand Duchy finally converted to Christianity in 1386, when Grand Duke Jogaila accepted baptism from Poland before his wedding to reigning Queen Jadwiga and coronation as King of Poland. However, the baptism did not stop the crusade as the Order publicly challenged sincerity of the conversion at the Papal court. Lithuania, together with its new powerful ally Poland, defeated the Order in the decisive Battle of Grunwald in 1410, which is often cited as the end of the Lithuanian Crusade. The final peace was reached by the Treaty of Melno (1422).
Document 8: History of Lithuania: The history of Lithuania dates back to settlements founded many thousands of years ago , but the first written record of the name for the country dates back to 1009 AD. Lithuanians, one of the Baltic peoples, later conquered neighboring lands and established the Grand Duchy of Lithuania in the 13th century (and also a short-lived Kingdom of Lithuania). The Grand Duchy was a successful and lasting warrior state. It remained fiercely independent and was one of the last areas of Europe to adopt Christianity (beginning in the 14th century). A formidable power, it became the largest state in Europe in the 15th century through the conquest of large groups of East Slavs who resided in Ruthenia. In 1385, the Grand Duchy formed a dynastic union with Poland through the Union of Krewo. Later, the Union of Lublin (1569) created the Polish–Lithuanian Commonwealth that lasted until 1795, when the last of the Partitions of Poland erased both Lithuania and Poland from the political map. Afterward, the Lithuanians lived under the rule of the Russian Empire until the 20th century.
Document 9: History of Lithuania (1219–95): The history of Lithuania between 1219 and 1295 concerns the establishment and early history of the first Lithuanian state, the Grand Duchy of Lithuania. The beginning of the 13th century marks the end of the prehistory of Lithuania. From this point on the history of Lithuania is recorded in chronicles, treaties, and other written documents. In 1219, 21 Lithuanian dukes signed a peace treaty with Galicia–Volhynia. This event is widely accepted as the first proof that the Baltic tribes were uniting and consolidating. Despite continuous warfare with two Christian orders, the Livonian Order and the Teutonic Knights, the Grand Duchy of Lithuania was established and gained some control over the lands of Black Ruthenia, Polatsk, Minsk, and other territories east of modern-day Lithuania that had become weak and vulnerable after the collapse of Kievan Rus'.
Document 10: Battle of Grunwald: The Battle of Grunwald, First Battle of Tannenberg or Battle of Žalgiris, was fought on 15 July 1410 during the Polish–Lithuanian–Teutonic War. The alliance of the Kingdom of Poland and the Grand Duchy of Lithuania, led respectively by King Władysław II Jagiełło (Jogaila) and Grand Duke Vytautas, decisively defeated the German–Prussian Teutonic Knights, led by Grand Master Ulrich von Jungingen. Most of the Teutonic Knights' leadership were killed or taken prisoner. Although defeated, the Teutonic Knights withstood the siege of their fortress in Marienburg (Malbork) and suffered minimal territorial losses at the Peace of Thorn (1411) (Toruń), with other territorial disputes continuing until the Peace of Melno in 1422. The knights, however, would never recover their former power, and the financial burden of war reparations caused internal conflicts and an economic downturn in the lands under their control. The battle shifted the balance of power in Central and Eastern Europe and marked the rise of the Polish–Lithuanian union as the dominant political and military force in the region.
Document 11: Vytautas: Vytautas (c. 1350 – October 27, 1430), also known as Vytautas the Great (Lithuanian: "   ", Belarusian: "Вітаўт Кейстутавіч (Vitaŭt Kiejstutavič)" , Polish: "Witold Kiejstutowicz" , Rusyn: "Vitovt", Latin: "Alexander Vitoldus") from the 15th century onwards, was a ruler of the Grand Duchy of Lithuania, which chiefly encompassed the Lithuanians and Ruthenians. He was also the Prince of Hrodna (1370–1382), Prince of Lutsk (1387~1389), and the postulated king of the Hussites.
Document 12: Daujotas: Daujotas was one of the 5 elder Lithuanian dukes mentioned in the peace treaty with Halych-Volhynia in 1219. In the same document Vilikaila is mentioned as brother of Daujotas which suggests that Daujotas was the older or perhaps more influential brother. The brothers are not mentioned in any other sources. Tomas Baranauskas, a modern Lithuanian historian, believes that Vilikaila and Daujotas might be sons of Stekšys, a Lithuanian duke killed in 1214.
Document 13: Lithuanian Civil War (1389–92): The Lithuanian Civil War of 1389–92 was the second civil conflict between Jogaila, King of Poland and Grand Duke of Lithuania, and his cousin Vytautas. At issue was control of the Grand Duchy of Lithuania, then the largest state in Europe. Jogaila had been crowned King of Poland in 1386; he installed his brother Skirgaila as ruler of Lithuania. Skirgaila proved unpopular and Vytautas attempted to depose him. When his first attempt to take the capital city of Vilnius failed, Vytautas forged an alliance with the Teutonic Knights, their common enemy – just as both cousins had done during the Lithuanian Civil War between 1381 and 1384. Vytautas and the Knights unsuccessfully besieged Vilnius in 1390. Over the next two years it became clear that neither side could achieve a quick victory, and Jogaila proposed a compromise: Vytautas would become Grand Duke and Jogaila would remain Superior Duke. This proposal was formalized in the Ostrów Agreement of 1392, and Vytautas turned against the Knights. He went on to reign as Grand Duke of Lithuania for 38 years, and the cousins remained at peace.
Document 14: Vaidotas: Vaidotas or Wojdat (fl. 1362) was a son of Kęstutis, Grand Duke of Lithuania. In reliable historical sources he is mentioned only twice: as defender of Kaunas Castle in 1362 and as ruler of Navahrudak. Due to very limited information, his life is subject to wide-ranging theories by historians.
Document 15: Sigismund Kęstutaitis: Sigismund Kęstutaitis (Lithuanian: "Žygimantas I Kęstutaitis" ; Polish: "Zygmunt Kiejstutowicz" ; c. 1365 – 20 March 1440) was the Grand Duke of Lithuania from 1432 to 1440. Sigismund was his baptismal name; Sigismund's pagan Lithuanian birth name is unknown. He was the son of the Grand Duke of Lithuania Kęstutis and his wife Birutė.
ground_truth: October 27, 1430