Complex sentence: Once in power he had Cossack leaders executed, and imprisoned because he perceived them as possible rivals, he had Vyhovsky, Ivan Bohun executed and had Yuri Khmelnytsky imprisoned.
Split into simpler sentences: Once in power he had Cossack leaders executed, and imprisoned because he perceived them as possible rivals. He had Vyhovsky, Ivan Bohun executed and had Yuri Khmelnytsky imprisoned.