Soldier of Fire - Amberlyn Holland

At the moment tyrannical King Aegron the Cruel was assassinated, each of his children was struck by a different dark curse.
Five years later, the Dark Kingdom is struggling to survive.

A hereditary curse has afflicted the Daesar Imperial family for generations and Princess Xadi will do whatever it takes to save her sister from dark magic.
Even join forces, temporarily, with the unacknowledged royal son of a rival kingdom.
With an unnatural fire burning in his eyes and demon madness invading his mind, Raneir has a curse of his own that he'll risk anything to break.
They both know the fragile truce will end the moment they find the ancient magic relic hidden within the Citadel.
Because only one curse can be broken.
But working together draws Xadi closer to Raneir than she ever imagined. And when their search leads them to a very real demon, more than one life is on the line.
...