Sebastian, Laura. Ash Princess (Series) #1

Series information from goodreads.com

Held captive by the brutal Kaiser since age six when she witnessed her mother's murder, Theodosia, called Ash Princess, is now sixteen and prepared to reclaim the throne by any means necessary

When her country was invaded, Theodosia's mother, the Fire Queen, was murdered before her eyes. The Kaiser had Theo crowned Ash Princess-- a title of shame to bear in her new life as a prisoner. For ten years a captive in her own palace, Theo has endured the relentless abuse and ridicule of the Kaiser and his court. One night the Kaiser forces her to do the unthinkable. With blood on her hands and all hope of reclaiming her throne lost, surviving is no longer enough. But she does have a weapon: her mind is sharper than any sword. And power isn't always won on the battlefield

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