Complex sentence: To escape execution Caris agrees to join the Kingsbridge nunnery, eventually becomes Prioress, forcing her out of any position to marry Merthin, who leaves Kingsbridge for Florence to pursue his building career.
Split into simpler sentences: To escape execution, Caris agrees to join the Kingsbridge nunnery. Her impending marriage to Merthin now made impossible, he leaves Kingsbridge for Florence to pursue his building career.