Obsidian. (Black Butler fanfic)

Saige McBlaire, 20-year-old Higher Flight agent, has been through all kinds of struggles. She has overcome fear, anger, the tragic loss of her right arm, and death. She has risen above these setbacks, and through intense training (and some lessons learnt through experience alone) has become one of the most talented assets in Higher Flight. She has been to universes besides her own, and even transcended dimensions to save a person's life. But all her missions and training could never have prepared her for the events that follow her 20th birthday. When Saige is sent to another universe to do undercover recon as a maid, things begin to happen to her... unexplainable things. And to complicate things more, a strange colleague is starting to get suspicious of her true motives.