Shadows on the Stars (Merlin Saga Series #10)

As peace returns to Avalon, Tamwyn, Elli, and Scree discover a terrifying new threat: The warlord Rhita Gawr has set out to conquer Avalon as well as mortal Earth. Racing against time, the friends embark on three separate quests. To succeed, they must solve Avalon's most elusive mysteries. And they will need to travel vast distances—both in their world and in their own hearts.