What does Harry do to defeat the Northern army?

Harry climbs one side of gap, noting she will return. At the top of the mountain, she finds her right arm, still numb, drawing Gonturan, as she calls out for Lady Aerin and Corlath. From the plain below, Jack and the others see a blinding white light. She screams out in the Old Tongue as the mountain begins to fall onto the Northern army below, crushing them to death and closing the pass.