Complex sentence: In ancient geography, especially in Roman sources, Dacia was the land inhabited by the Dacians or Getae as they were known by the Greeks--a branch of the Thracians north of the Haemus range.
Split into simpler sentences: In ancient geography, especially in Roman sources, Dacia was the land inhabited by the Dacians. The Greeks referred to them as the Getae, which were specifically a branch of the Thracians north of the Haemus range.