Complex sentence: Slavs settled in the Balkans in the 6th and 7th centuries, where they subsequently absorbed the local population (Illyrians, Thracians, Dacians, Romans, Celts).
Split into simpler sentences: Slavs settled in the Balkans in the 6th and 7th centuries. They mixed with the local population (Illyrians, Thracians, Dacians, Romans, Celts).