Complex sentence: The Zaporozhian Cossacks, who lived on the Pontic-Caspian steppe below the Dnieper Rapids (Ukrainian: "za porohamy";), became a well-known group whose numbers increased greatly between the 15th and 17th centuries.
Split into simpler sentences: The Zaporozhian Cossacks, who lived on the Pontic-Caspian steppe below the Dnieper Rapids (Ukrainian: "za porohamy";), became a well-known group. Cossack numbers increased greatly between the 15th and 17th centuries.