Short answers, pulled from the story.
The magnates of Poland and Lithuania, known in Polish as magnateria, were the wealthiest and most politically powerful segment of the noble class, called szlachta, in the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania. They emerged as a distinct social group around the 16th century and dominated the Polish-Lithuanian Commonwealth until the Third Partition of Poland in 1795.
The term królewięta, meaning "little kings," referred to magnates who controlled vast eastern estates known as latifundia, which included private cities and private armies. These magnates operated with such independence from central authority that the comparison to kings was seen as apt by contemporaries.
Famous magnate families from the Crown of Poland included the Czartoryski, Potocki, Zamoyski, Ostrogski, Koniecpolski, and Wiśniowiecki families, among others. In the Grand Duchy of Lithuania, notable families included the Radziwiłł, Sapieha, Pac, and Tyszkiewicz families.
No. "Magnate" was not an official legal title in the Polish-Lithuanian Commonwealth. All members of the szlachta were equal under the law, and titles such as prince or count were largely forbidden, with limited exceptions tied to the 1569 Union of Lublin. Magnate status was a social and economic distinction based on wealth and political influence, not a formal legal category.
Magnate influence rose sharply from the late 16th century, and scholars identify the second half of the 17th century as the period of magnate oligarchy, when political life in the Commonwealth was, as historian Norman Davies wrote, "reduced to the feuds, fortunes, and the follies of a few families."
The Third Partition of Poland in 1795 ended the Commonwealth and removed the political institutions through which the magnates exercised power. The class survived in a diminished form under foreign rule but was finally extinguished by the Second World War and the communist-ruled People's Republic of Poland, which nationalized their estates.