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

Context paragraphs:
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.

Family of Gediminas: The family of Gediminas is a group of family members of Gediminas, Grand Duke of Lithuania (ca. 1275–1341), who interacted in the 14th century.  The family included the siblings, children, and grandchildren of the Grand Duke and played the pivotal role in the history of Lithuania for the period as the Lithuanian nobility had not yet acquired its influence.  Gediminas was also the forefather of the Gediminid dynasty, which ruled the Grand Duchy of Lithuania from 1310s or 1280s to 1572.

Andrei of Polotsk: Andrei of Polotsk (Belarusian: Андрэй Альгердавіч , Lithuanian: "Andrius Algirdaitis" Polish: "Andrzej Olgierdowic" , ca. 1325 – 12 August 1399, in the Battle of the Vorskla River) was the eldest son of Algirdas, Grand Duke of Lithuania, and his first wife Maria of Vitebsk.  He was Duke of Pskov (through his deputy Yuri, 1342–1348) and Polotsk (1342–1387).  As the eldest son of the Grand Duke, Andrei claimed his right to the throne after his father's death in 1377.  Algirdas left Jogaila, his eldest son with his second wife Uliana of Tver, as the rightful heir.  Andrei's rivalry with Jogaila, Grand Duke of Lithuania and later King of Poland, eventually led to his demise.

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.

Mikhailo Olelkovich: Mikhailo Aleksandrovich Olelkovich (executed on August 30, 1481 in Vilnius) was a noble from the Olelkovich family of the Grand Duchy of Lithuania.  He was the brother of Prince Simeon Olelkovich of Kiev and cousin of Grand Prince Ivan III of Moscow.  Mikhailo was allegedly involved both in bringing the Judaizer Heresy to Novgorod and the failed defection of the city to the Grand Duchy of Lithuania in 1471.  He also organized a coup against Casimir IV Jagiellon, King of Poland and Grand Duke of Lithuania, but was discovered and executed in 1481.  Mikhailo's son Semen continued the family line.

Union of Horodło: The Union of Horodło or Pact of Horodło was a set of three acts signed in the town of Horodło on 2 October 1413.  The first act was signed by Władysław II Jagiełło, King of Poland, and Vytautas, Grand Duke of Lithuania.  The second and third acts were composed by the Polish nobility (szlachta) and Lithuanian boyars respectively.  The union amended the earlier Polish–Lithuanian unions of Krewo and Vilnius–Radom.  Politically, Lithuania received more autonomy as, after the death of Vytautas, the Lithuanian nobles could choose another Grand Duke instead of passing the title to Władysław II Jagiełło or his heir.  However, culturally, Lithuania and Poland grew closer.  Lithuania adopted Polish institutions of castellans and voivodes. Catholic Lithuanian nobles and church officials were granted equal rights with the Polish nobles and clergy.  Forty-seven selected Lithuanian nobles were adopted by Polish families and granted Polish coats of arm.  Thus the union signified the beginnings of Polonization of Lithuanian culture and the rise of the Lithuanian nobility.  It was one of the major steps towards the modernization and Europeanization of Lithuania.

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.

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.

Michael Glinski: Michael Glinsky (Lithuanian: "Mykolas Glinskis" , Russian: Mikhail Lvovich Glinsky , Polish: "Michał Gliński" ; 1460s – 24 September 1534) was a noble from the Grand Duchy of Lithuania of distant Tatar extraction who was also a tutor of his nephew, Ivan the Terrible.  As a young man, Glinsky served in the court of Emperor Maximilian I and earned distinction for his military service.  Around 1498 he returned to Lithuania and quickly rose in power and wealth, angering local nobles.  Just after commanding the victorious Battle of Kletsk against the Crimean Khanate in August 1506, he was accused of conspiracy against the deceased Grand Duke Alexander Jagiellon and lost all his wealth.  Glinsky began an armed rebellion against Sigismund I, the new Grand Duke.  The rebellion was unsuccessful and Glinski retreated to the Grand Duchy of Moscow, where he served Vasili III of Russia.  When the Muscovite–Lithuanian Wars renewed in 1512, Glinsky was instrumental in helping Moscow to capture Smolensk, a major trading center.  However, he was not rewarded with the regency of the city.  Angered, he planned to betray Vasili III, but the plot was discovered and he was imprisoned for 12 years.  He was freed after his niece Elena Glinskaya married Vasili III in 1526.  Before his death in 1533, Vasili appointed Elena and Glinski as protectors of his underage sons Ivan and Yuri.  Elena disapproved of Glinsky's influence in the state and had him sent to prison, where he soon died of starvation.

Daumantas of Lithuania: Daumantas or Dovmont was the Grand Duke of Lithuania in 1282–1285.  Daumantas is mentioned in chronicles only once and, in absence of any other evidence, is presumed to be a short-ruled Grand Duke who inherited the title after Traidenis' death in 1281 or 1282.  It is assumed that Daumantas was succeeded by Grand Duke Butegeidis.  Relationships between Traidenis, Daumantas, and Butegeidis are unknown.

Supporting paragraphs: Lithuanian Civil War (1389–92) Vytautas
Answer: October 27, 1430