Complex sentence: Medieval Duklja and its successor-state Zeta were part of the larger Serbian realms that presently are part of southern Dalmatia, Bosnia and Herzegovina, Montenegro, Serbia and Malesija.
Split into simpler sentences: Medieval Duklja and its successor-state Zeta were part of the larger realms occupied by Serbs. Those lands presently are part of southern Dalmatia, Bosnia and Herzegovina, Montenegro, Serbia and Malesija.