Complex sentence: Pen and Grianne return to Paranor and after the trap is sprung, Grianne unleashes the power of her Wishsong upon it, not only utterly destroying the triagenel, but also obliterating one of the walls of the room.
Split into simpler sentences: Pen and Grianne return to Paranor and, after the triagenel is sprung, Grianne unleashes the power of her Wishsong upon it. This not only utterly destroying the triagenel, but also obliterating one of the walls of the room.