The Tyrant's Law. Book 3 of the Dagger and the Coin

The great war cannot be stopped. The tyrant Geder Palliako begins a conquest aimed at bringing peace to the world, though his resources are stretched too thin. When things go poorly, he finds a convenient target among the thirteen races and sparks a genocide. Clara Kalliam, freed by having fallen from grace, remakes herself as a 'loyal traitor' and starts building an underground resistance movement that seeks to undermine Geder through those closest to him.
Cithrin bel Sarcour is apprenticing in a city that's taken over by Antea, and uses her status as Geder's one-time lover to cover up an underground railroad smuggling refugees to safety. And Marcus Wester and Master Kit race against time and Geder Palliako's men in an attempt to awaken a force that could change the fate of the world.