Timeless, book 3: AbyssLaura Kreitzer2011

Gabriella is falling to pieces. The aftermath of the Soul Stalker's death weighs heavily on her shoulders. Andrew tries everything he can to help her fight off the new demon inside her, the one who wants to cause chaos and commit murder. When symbols begin to ink her skin, they realize there is something greater going on in Gabriella's soul. A war. But there is another war waging in the Angelic world. Angels are being picked off one-by-one. Gabriella is faced with the dilemma of picking a side. Phantoms entice her with a new life while the Empyrean Guard offer a heavenly death. Our heroine has another idea, though. She seeks the Fallen in the Abyss. Shadows are Illuminated, an army is formed, and a surprise appearance by someone thought dead helps Gabriella fight the darkness inside. There can't be shadows without light, but the two together collide in the explosive third installment of the Timeless Series.