The last Hochmeister of the Teutonic order, Albert, converted to Protestantism and became the Duke of Prussia, secularizing the lands owned by the order. He swore a personal oath to the king of Poland in 1525 who confirmed his legitimacy over the former Ordensland. Albert ruled as Duke until 1568.

The son of Albert, Albert Frederick, ruled over the Duchy until 1618 (the same year the 30 Years War broke out). Albert Frederick did not have any male children, so the Elector of Brandenburg, John Sigismund, inherited the Duchy of Prussia because of his marriage to Albert Frederick's daughter the Duchess Anna. Thus Brandenburg-Prussia was created. John Sigismund's rule over both territories was brief (he died in 1619), passing to his son Albert Frederick.

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Albert Frederick probably would not have survived the Thirty Years War without the Prussian territories, as he was forced to flee there during the conflict. Brandenburg was absolutely devastated by the war (60% of the population was killed or displaced), and the Prussian territories were a critical source of wealth for the Elector after the Thirty Years War ended.

Basically Brandenburg was totally ruined, and the Elector would have to important other German and polish immigrants to the territory after the war. When German ethnologists would come to the Brandenburg territory in the 19th century, they would find little in the way of indigenous mythology or folklore to record. This is because the pre-1618 population had been wiped out, and there had been almost no continuity with their successors.

If you are interested in Prussia, go read Iron Kingdom, it's fantastic.