Complex sentence: The Massagetae are known primarily from the writings of Herodotus who described the Massagetae as living on a sizeable portion of the great plain east of the Caspian Sea.
Split into simpler sentences: The Massagetae are known primarily from the writings of Herodotus. Herodotus described the Massagetae as living on a sizeable portion of the great plain east of the Caspian Sea.