Complex sentence: Upon the death of magnate Stanisław Koniecpolski, an advocate of fair treatment of Cossacks, his successor Aleksander redrew the maps of his possessions and laid claim to Khmelnytsky's estate, claiming it as his.
Split into simpler sentences: Upon the death of magnate Stanisław Koniecpolski, an advocate of fair treatment of Cossacks, his successor Aleksander redrew the maps of his possessions. He laid claim to Khmelnytsky's estate, claiming it as his.