Knight's Absolution (Knights of Hell #5)

Loyal and deadly, Gunner has fought tirelessly alongside his brothers in the war between Heaven and Hell for the one reward he craves: a beloved mate of his own. But darkness snatched her away. Now she haunts his dreams, and his longing for a female lost to him forever is tormenting him to the edge of sanity.
Luna risked everything to help the brutal warrior who should be her enemy—and lost. Now trapped in a cell and punished relentlessly for her betrayal, her wish for death is bested only by an intense desire for the man she cannot have. When he finds and rescues her, it awakens a part of her she never knew existed.
But the monster who held her captive will stop at nothing to get Luna and her powers back, and those she loves most are in terrible danger...