Wood versus steel. Magic versus mechanics. The peace of Elara shatters when the Ferros Empire invades to process the forests into fuel. A mysterious frequency robs Queen Aelia and the wind spirit Alarion of their powers, leaving them defenseless against the gigantic harvesters. Their only chance lies in the heart of the enemy: the Iron Citadel. There, they encounter Aelia's sister, Lira, whom she believed to be dead-now a deadly cyborg. To overthrow the world of machines, Aelia must recognize that nature has a dark side: she must not summon life, but rust, to break the steel.