Flamecaster / Cinda Williams Chima.

Enhanced descriptions from Syndetics:

A burning vengeance

Adrian sul'Han, known as Ash, is a trained healer with a powerful gift of magic--and a thirst for revenge. The son of the queen of the Fells, Ash is forced into hiding after a series of murders throws the queendom into chaos. Now Ash is closer than he's ever been to killing the man responsible, the cruel king of Arden. As a healer, can Ash use his powers not to save a life, but to take it

A blood-based curse

Abandoned at birth, Jenna Bandelow was told the mysterious magemark on the back of her neck would one day make her a target. But when the King's Guard launches a relentless search for a girl with a mark like hers, Jenna assumes that it has more to do with her role as a saboteur than any curse. Though Jenna doesn't know why she's being hunted, she knows that she can't get caught.

Destiny's fiery hand

Thrown together by chance and joined by their hatred of the ruthless king, Ash and Jenna will come to rescue each other in ways they cannot yet imagine.

Set in the world of the acclaimed Seven Realms series, a generation later, this is a thrilling story of dark magic, chilling threats, and two unforgettable characters walking a knife-sharp line between life and death.