What are the motifs in The Wicked Day by Mary Stewart?

The power of evil is a recurring idea in the story. In this story, there is great evil and a thirst for revenge. Two sisters spend their entire lives manipulating people and circumstances, poisoning minds and bodies, and using their beauty and influence to crush the dream of Camelot. The power Arthur represents to them is unacceptable, and they devote every waking moment to perfecting their black arts so they can bring him down.