A Dawn of Onyx by Kate Golden

Captured by a king of darkness. Forced to find the light within.
\n
\nArwen Valondale never expected to be the brave one, offering her life to save her brother?s. Now she?s been taken prisoner by the most dangerous kingdom on the continent, and made to use her rare magical abilities to heal the soldiers of the vicious Onyx King.
\n
\nArwen knows better than to face the ancient, wicked woods that surround the castle on her own, which means working with a fellow prisoner might be her only path to freedom. Unfortunately, he?s as infuriating as he is cunning?and seems to take twisted pleasure in playing on Arwen?s deepest fears.
\n
\nBut here in Onyx Kingdom, trust is a luxury she can?t afford.
\n
\nTo make it out of enemy territory, she?ll have to navigate back-stabbing royals, dark magic, and dangerous beasts. But untold power lies inside Arwen, dormant and waiting for a spark. If she can harness it, she just might be able to escape with her life?and hopefully, her heart.