Complex sentence: Finally, Grianne, having negotiated an arms treaty calling for the elimination of all crystal-based weapons research, retires as Ard Rhys and travels with Pen to try to save Pen's girlfriend Cinnaminson, who was transformed into an Aeriad spirit of the Tanequil tree.
Split into simpler sentences: Finally, Grianne, having negotiated an arms treaty calling for the elimination of all crystal-based weapons research, retires as Ard Rhys. She travels with Pen to try to save Pen's girlfriend Cinnaminson, who was transformed into an Aeriad spirit of the Tanequil tree.