Prophecy Betrayed

It was said that when humanity finally fell into darkness, there would be one who would come from the light to lead humanity out of it. This is the Prophecy. This is Ariel, humanity's great savior. She's spent her whole life fighting demons and spiriting human souls to safety, doing what Fate decided for her. However, when she meets a particular demon she discovers that everything she has ever known has been a lie, and she has been used as a tool to end the human race--not save it.