The Whim of the Dragon (The Secret Country Trilogy, Vol. 3)

Three things have the power to destroy the Secret Country: the Border Magic, the Crystal of Earth, and the whim of the dragon. The cousins have faced the first two; now they face the third. The Country's most trusted counselors know that the five are impostors, but no one knows who has been playing with their destinies. They must find and speak with Chryse the unicorn and Belaparthalion the dragon in order to learn the truth.