Complex sentence: George of Poděbrady, the Hussite ruler of Bohemia, promised to make Vladislaus his heir in 1468, because he needed Casimir IV's support against the rebellious Catholic noblemen and their ally, Matthias Corvinus, king of Hungary.
Split into simpler sentences: George of Poděbrady, the Hussite ruler of Bohemia, offered to make Vladislaus his heir in 1468. Poděbrady needed Casimir IV's support against the rebellious Catholic noblemen and their ally, Matthias Corvinus, king of Hungary.