Complex sentence: Contrary Mary agrees to marry him in exchange for Alan's pardon, but Barnaby marries her, denounces Alan again, and dies, after drinking a wine glass filled with poison meant for Alan.
Split into simpler sentences: Contrary Mary agrees to marry Barnaby in exchange for Alan's pardon, but after he marries her, Barnaby denounces Alan again. Barnaby dies after drinking a wine glass filled with poison meant for Alan.