Complex sentence: Some time later, Khan Boris concluded an alliance with the Great Moravian Knyaz Rastislav (846-870) instigated by the King of the West Franks, Charles the Bald (840-877).
Split into simpler sentences: Some time later, Khan Boris concluded an alliance with the Great Moravian Knyaz Rastislav (846-870). The catalyst for this move was the King of the West Franks, Charles the Bald (840-877).