A Mark of Death and Darkness. The Slayer's Descent, #1

Raised to hunt monsters. Destined to become one. Rayna Quinn hunts monsters for a living-vampires, demons, anything with fangs and a pulse. Trianed by the Bureau to believe that supernaturals are evil and humans need protection, her world has always been simple and brutal. Kill the things that go bump in the night before they kill someone else. Clean. Black-and-white. But when a high-stakes mission forces Rayna into the supernatural nightclub Death's Doorway, she crosses paths with Azrael Malachi-an ancient, infuriatingly powerful being who challenges everything she's been taught.
And when Rayna's sister Piper is kidnapped, the truth hits even harder: the only person who can help her rescue Piper is the same creature she swore to destroy. As the hunt spirals into a collision of secrets, loyalties, and awakening power, Rayna is forced to face an impossible truth about who she is and what she was meant to become. With time running out, she must choose between the lie that kept her safe or embrace the monstrous truth. Enter a world of death, destiny, and explosive magic where nothing is as it seems.