Complex sentence: After capturing Sirmium, the Avars began to rapidly occupy the Balkans, taking the cities of Augustae, Singidunum, and Viminacium in 583, and a further eight of nine cities were taken in 586.
Split into simpler sentences: After capturing Sirmium, the Avars began to rapidly encroach into the Balkans. The Avars took the cities of Augustae, Singidunum, and Viminacium in 583, and a further eight of nine cities in 586.