Her Avenging Angel by Felicity Heaton

Once a proud angel of Heaven, Nevar is now a servant of Hell, bound to a new master—the King of Demons. Consumed by darkness and driven to seek revenge, he set in motion a series of events that awakened the Great Destroyer, a force that will bring about the apocalypse. Now, he is the creature’s master and the fate of our world rests in the hands of an angel with only darkness in his heart.
Lost in the mortal realm without any recollection of how she came to be there, Lysia is only aware that she has survived a great battle. When she stumbles into a demon bar, she finds more than a chance to discover what happened to her—she finds a dark and deadly angel warrior who stirs fire in her veins and awakens soul-searing passion she cannot deny.
With the mounting threat of the Great Destroyer, the forces of Heaven and Hell against him, and a band of dangerous angels intent on capturing Lysia on his heels, can Nevar protect the beautiful woman who is light to his darkness and find the strength to save the world?