The Serpent and the Wolf by Rebecca Robinson

All her life, Vaasa Kozár has been sharpened into a blade. After losing her mother—her only remaining parent—to a mysterious dark magic that has since awakened within her, Vaasa is certain death looms. So is her merciless brother, who aims to eliminate Vaasa as a threat to his crown. In one last political scheme, he marries her off to Reid of Mireh, a ruthless foreign ruler, in hopes that he can use her death as a rallying cry to finally invade Reid’s nation. All Vaasa has to do is die.