Immortals: The Darkening by Robin T. Popp, (Immortals Series, Book 2) From Books in Motion.com

While Lexi, a powerful witch-werewolf and bounty hunter, tracks criminals through Manhattan, she runs across Darius, an Immortal who has landed in her city after the failure of The Calling spell. Attracted to the sexy warrior, but struggling to deal with the raging needs of the wolf inside her, Lexi battles her own desires as she and Darius race to foil a plot to overrun the city with Death Magic. Time is running out for them to reach the other Immortals and stop the deadliest threat the world has known.