Like all of her magical food, it was highly addictive, thus making them easy for her to control. With it, she was able to get information about Edmund's siblings from him, as well as learn about the Faun Tumnus who had helped Lucy escape,
After Edmund breaks the wand of Jadis, he is stabbed forcing Peter to face Jadis in a one on one battle. Aslan returns, brought his army and banishes the White Witch to finish the battle!