Complex sentence: Ladislaus IV's great-nephew, Charles I, who was a scion of the Capetian House of Anjou, captured the oligarchs' most castles in the 1310s and 1320s, which again secured the preponderance of the royal demesne.
Split into simpler sentences: Ladislaus IV's great-nephew, Charles I, who was a scion of the Capetian House of Anjou, restored royal power in the 1310s and 1320s. He captured the oligarchs' castles, which again secured the preponderance of the royal demesne.