Complex sentence: The term "Balkar" is said to be derived from "Bolgar" or "Bulgar", the Balkars supposedly being Bulgars who lived in Onoghur and Great Bulgaria and who remained in the Caucasus as the others migrated to the Balkans and Middle Volga.
Split into simpler sentences: The term "Balkar" is said to be derived from "Bolgar" or "Bulgar". The Balkars were those Bulgars who lived in Onoghur and Great Bulgaria and who remained in the Caucasus as the others migrated to the Balkans and Middle Volga.