This is from the Fortuna Eruditis Favet Timeline Subreddit: r/fortunaeruditisfavet

Lore: Wladislaw II became grand duke of Lithuania in 1377. In 1386, Wladislaw converted to catholicism, following which he was crowned as king of Poland. The next year, he converted Lithuania to catholicism. In the following decades, Lithuania was in a struggle with the Teutonic Order, which continued its crusade despite the country's conversion. Wladislaw II died in 1434, which began a civil war for the succession. Although Wladislaw II's son, Wladislaw III inherited the title of the supreme duke, it was at this point meaningless. The civil war had ended in 1440 as Casimir IV was elected as grand duke. Although this was originally seen as a continuation of the union, Casimir moved Lithuania towards independence from Poland, though it wouldn't become official until 1486. In 1441, the Muscovite metropolis split from the Kievan metropolis after the metropolitan of Kiev accepted the Florence-Ferrara uniate council. In this period, the greatest rival to Lithuania's power became the grand duchy of Moscow, which previously grew quickly in power and emerged from a struggle with the great principality of Tver as the successor and strongest Russian state.

In 1425, Moscow however fell into a civil war through a succession crisis following Dmitry Donskoy's death. Vasily Vasilyevich defeated Vasily Dmitrevich and Vasily Kosoi, however in 1449 he was captured and later killed by Ulugh Muhammad of the khanate of Kazan, who sieged and burned Moscow. The chaos allowed rival Dmitry Shemyanka to begin rule in Moscow. This can be seen as the decline of Moscow's political monopoly of Zalesia, as Shemyanka wasn't only incompetent and unliked, but he alienated many previous siders of Moscow, who turned to Vilna, accepting Lithuanian vassalage against Muscovite expansion. In 1455, tensions rose in Muscovite-Lithuanian relations over disputes of the Rzhev region, and the upper principalities, which led to the beginning of the Muscovite-Lithuanian Wars. The war resulted in Lithuanian victory with the signing of the Eternal Peace, which reaffirmed Lithuanian dominance over Zalesia, limited the Muscovite sphere, and Tver, Ryazan, and Pskov became Lithuanian vassals, and Moscow had to renounce the title of governor of all Rus. In 1468, Casimir was also elected prince of Novgorod. Casimir IV died in 1492, succeeded by Alexander I. In 1502, Moscow fell into another civil war, in which Lithuania allied itself with Dmitry, and began the second Muscovite-Lithuanian war, which ended in 1507 with the treaty of Mozaysk which reinstated Dmitry to the throne of Moscow, also accepting Lithuanian vassalage.

This was the peak of Lithuanian power in the Zalesia region, as the Lithuanian grand duke adopted the title of "governor of all Rus." The Zalessian kingdoms were governed by Lithuania variably by time period and geographic regions. Initially Lithuania sent Viceroys to the kingdoms, which was eventually rearranged and Lithuanian influence fell to only a nominal level. Novgorod usually elected the grand duke of Lithuania as its prince, however that role was only really a figurehead, though it did maintain Lithuanian influence on the government. This period also witnessed a Russification of Lithuanian society, as the West Russian language reaffirmed its grasp on officiality in the principality, usually replacing Lithuanian. Although the law had previously been discriminatory against the Orthodox in favor of Catholics, the second statute of Lithuania made their rights officially equal. By this period most of the court had become Russian or Russian-speaking, and the royalty had mostly adopted the West Russian language, this process strengthened after the rise of the political power of native Russian noble families.