The Wicked. Dark Sorcery, #3

Rhiannon Castle is a San Francisco witch whose Coven sisters know nothing of the shadows that lurk inside her. Rhiannon hides the truth from everyone she loves-and most of all Keir, a rugged warrior who strides into Rhiannon's life and awakens her to a reckless desire. Keir's Fae brethren are the only family he has ever known. He trusts no one-until he is sent to San Francisco and meets Rhiannon. She is the one woman with enough fire to tame the fierce warrior.
Every encounter convinces Keir that she belongs with him-even though Rhiannon knows her powers could put him in danger. From the depths of Underworld rises a new threat. When a demon goddess unleashes her evil upon our world, Rhiannon's secret could prove the ultimate weapon in this epic battle-or destroy the only man she has ever loved.