Complex sentence: After Kubrat's death, his empire was divided when his appointed heir Batbayan submitted to the Khazars of Kubrat's second son Kotrag who settled Batbayan's army at the confluence of the Volga and Kama rivers where they founded a Khanate known as Volga Bulgaria.
Split into simpler sentences: After Kubrat's death, his empire was divided when his appointed heir Batbayan submitted to the Khazars. At this, some Kutrigurs, led by Kubrat's second son Kotrag, migrated to the confluence of the Volga and Kama rivers and founded a Khanate known as Volga Bulgaria.