Complex sentence: Soon after his election, he launched a unsuccessful campaign against Ulrich II, Count of Celje, who administered Slavonia with the title ban (which he had arbitrarily adopted) and refused to renounce of it in favor of Hunyadi's appointee.
Split into simpler sentences: Soon after his election, he launched an unsuccessful campaign against Ulrich II, Count of Celje. Count Ulrich administered Slavonia with the title ban (which he had arbitrarily adopted) and refused to renounce of it in favor of Hunyadi's appointee.