Complex sentence: In late 966 or early 967, he sent the "patrikios" Kalokyros, a citizen of Cherson, as his ambassador to Sviatoslav, ruler of the Rus' with whom the Byzantines had long maintained close relations and were bound by treaty.
Split into simpler sentences: In late 966 or early 967, he dispatched the "patrikios" Kalokyros, a citizen of Cherson, as his ambassador to Sviatoslav, ruler of the Rus'. The Rus' were a people with whom the Byzantines had long maintained close relations and were bound by treaty.