Eastern Hungarian Kingdomball was a Magyar vassal in Transylvania.
The Eastern Hungarian Kingdom was formed in 1526 after the Kingdom of Hungary lost to the Ottoman Empire at the Battle of Mohács and their king was killed. After the war, the Ottomans did not annex any land and the Habsburgs claimed the land for their kingdom. Many Hungarians did not like this and tried to have their own kingdom.
The Habsburgs held the western lands of the former Kingdom of Hungary while the East Hungarian Kingdom had the eastern lands. They had a rivalry and had a couple wars. In 1570, King John II signed a treaty with the Habsburgs and the Eastern Hungarian Kingdom became the Principality of Transylvania.