Complex sentence: After George Frederick's death in 1605, the Polish king Zygmunt Waza appointed Joachim Frederick as regent, and then in 1608, the new Elector of Brandenburg, John Sigismund, who also became Duke of Prussia after Albert Frederick's death in 1618.
Split into simpler sentences: After George Frederick's death in 1605, the Polish king Zygmunt Waza appointed Joachim Frederick as regent, and then in 1608, the new Elector of Brandenburg, John Sigismund. The latter became Duke of Prussia after Albert Frederick's death in 1618.