Shard in the Mirror. Crimson Realm Chronicles

Enslaved as one of Cewrick's feared black dragons, for centuries Taylith had been forced to do the evil sorcerer's bidding. Finally free of the shackles of slavery, Taylith is enlisted by King Biryn as a member of his elite team. Plagued with visions of an impending war and the return of the black dragon he once was, Taylith must find a way to tell his lifemate, Laura, that he was the creature that had captured her and delivered her into the hands of the enemy.
To keep Laura safe and save her sister from Zohmes' clutches, he must allow the god to change him back into the feared creature he once was.