Celestial Blight - The Farsian Trilogy, #1

In the Kingdom of Farsia, everyone has access to magic, but magic comes with a cost. Some pay for it with hunger or thirst, while the unlucky few must pay with their souls. Zemfira has spent her whole life in her village looking forward to unlocking her powers, until she learns she is one of the accursed: a Soulbreaker. Meanwhile at the capital, the royal bodyguard Volakles awaits execution for his failures.
His only chance to survive is to join the secret police, and dedicate himself to eradicating Soulbreakers. His first target is a girl named Zemfira. Soulbreakers aren't the only threat to the kingdom. Foreign armies plunder the western provinces and dark creatures prowl the twilight. Zemfira alone can stop the tide of chaos-but only if she's willing to pay the price.