Complex sentence: His nephew and successor, Justin II, played off the Avars against the Gepids and later against the Slavs, but that only allowed the Avar Khaganate to become a more powerful threat than the Gepids and Slavs.
Split into simpler sentences: His nephew and successor Justin II played off the Avars against the Gepids and later on against the Slavs. But this only allowed the Avar Khaganate to become a more powerful threat than the Gepids and Slavs.