Complex sentence: Many among the Romanised and Hellenised Paeonian, Illyrian and Thracian population of Macedonia were assimilated by the Slavs, but pockets of tribes that fled to the mountains remained independent.
Split into simpler sentences: Many among the Romanised and Hellenised Paeonian, Illyrian. Macedonian and Thracian population of Macedonia were assimilated by the Slavs, but pockets of tribes that fled to the mountains remained independent.