The Tinkerer's Daughter Series

Steam technology and spring-powered engines have evolved since the discovery of Blackrock Steel, which can store vast amounts of energy, but word of this incredible resource has spread far and wide. When a horde of barbarian warriors known as Vangars sweep across the Frigid Sea in massive airships fueled by precious but dwindling supplies of black tar-like oil, they are determined to learn the secrets of Blackrock Steel at any cost. They assault the capital city, slaying King Ryshan and the entire royal family, and then ravage the countryside.

Many years have passed since Breeze Tinkerman vanished in her spring-powered plane over the northern Wastes while searching for the fabled lost city in the ice, but her daughter River is more than happy to carry on her legacy. The Vangar overlords have turned Astatia into a slave state, forcing citizens to work in their mines and factories, stealing Astatia's resources for their own greedy ends while living safely high above in the sky-city known as Juntavar.