Who is Lady Oonagh from Daughter of the Forest and what is their importance?

Lady Oonagh is one of the novel's antagonists. She an evil woman, who places Lord Colum under a spell that tricks him into marrying her. It is Lady Oonagh, who places a curse on the brothers that turns them into swans, and only Sorcha can reverse the curse. As a result, Lady Oonagh does everything possible to locate Sorcha. Fortunately, despite many sacrifices, Sorcha manages to reverse the curse and return order to Sevenwaters.