well the slavs migrated in the balkans that were ruled by the byzantine empire, so the initial nobility were greek, of course in time slavic leader got their own titles but, up until the 20th century, a large portion of the clergy was greek as well, and unlike catholic priests, orthodox ones can and must marry, so a good portion of serbs in serbia have greek ancestry, there is even an assimilated ethnic group called cincari who are just serbified greeks
Serbia