What is the theme in The Wicked Day by Mary Stewart?

There is a continuing theme of questioning a fate someone else has decided for us. Mordred lives his entire adult life with Merlin's prophecy of doom hanging over his head. Almost at the same moment Mordred learns he is the son of the High King of the country, he learns his mother is a liar, that he was born of an incestuous union, and that the most famous wizard in the land has predicted that he, Mordred, will be the downfall of his father's kingdom. This gives Mordred a great deal to consider as he plans his future.