Complex sentence: Upon return to Subotiv, Khmelnytsky took over the running of his father's estate and became a registered Cossack in the Chyhyryn Regiment where he later became a pysar (a historical officer title among cossacks).
Split into simpler sentences: Upon return to Subotiv, Khmelnytsky took over the running of his father's estate and became a registered Cossack in the Chyhyryn Regiment. He was later promoted to pysar (a historical officer title among cossacks).