Books like Sweet Ruin by Kresley Cole

Immortals After Dark #15 An immortal assassin is caught between desire and duty... A foundling raised in a world of humans Growing up, orphaned Josephine didn’t know who or what she was—just that she was “bad,” an outcast with strange powers. Her baby brother Thaddeus was as perfect as she was flawed; protecting him became her entire life. The day he was taken away began Jo’s transition from angry girl... to would-be superhero... to enchanting, ruthless villain. A lethally sensual enforcer on a mission A threat to the Møriør has brought archer Rune the Baneblood to the mortal realm to slay the oldest living Valkyrie. Whether by bow or in bed, he never fails to eliminate his target. Yet before he can strike, he encounters a vampiric creature whose beauty conceals a black heart. With one bite, she pierces him with aching pleasure, taking his forbidden blood - and jeopardizing the secrets of his brethren. A boundless passion that will lead to sweet ruin... Could this exquisite female be a spy sent by the very Valkyrie he hunts? Rune knows he must not trust Josephine, yet he’s unable to turn her away. Despite his millennia of sexual conquests, he can’t ignore the unfamiliar longing she arouses deep within him. When Jo betrays the identity of the one man she will die to protect, she and Rune become locked in a treacherous battle of wills that pits ultimate loyalty against unbridled lust.

First publish date: 2015