Complex sentence: Since 920's the Byzantine Empire was considerably weakened so Emperor Romanos I gave management over Dalmatia to the Croatian King Tomislav who needed it in order to connect the church authorities in Croatia and Dalmatia so he could easily integrate Dalmatia into his Kingdom.
Split into simpler sentences: Since 920's the Byzantine Empire was considerably weakened so Emperor Romanos I gave management over Dalmatia to the Croatian King Tomislav. Tomislav needed to connect the church authorities in Croatia and Dalmatia so he could easily integrate Dalmatia into his Kingdom.