Oathbreaker — J. Kathleen Cheney

The chief of the Oathbreakers, Amal Horn is one of only a handful of people aware of the true power the abandoned underground Fortress of Salonen holds. The Cince Empire wants its secrets, though, and will do anything to get someone inside. When the Horn find a stranger trespassing on the glacier below it, they realize the Cince have formulated a new plan of attack. Now the Horn Family must decide whether to wake the sleeping Fortress so it can defend itself against the Cince…or kill it forever.