Lost Truth Truth Series, Book 4.

Alissa is steadily advancing in her magical studies with the last surviving Master at the legendary Hold. But she's haunted by dreams-dreams revealing that other Masters have survived. Alissa's dreams lead her to a distant island where she and a rescue party discover countless Masters, alive and part of a flourishing magical society. But instead of being welcomed, Alissa finds that she herself may need to be rescued. The Masters' leader, Keribdis...