Breaking Dawn by Stephenie Meyer

Life is hard, especially when you are a ghost. Ari Brooks, a 'woman' caught between two worlds, that of the living and that of the dead, finds herself slowly regressing into the person she was, when she had a body.

Hidden in her memories are the secrets to tracking down the last of Felecia's kind, the Greats; powers that have ruled since the beginning of time, now blending into the world around them, unnoticed. Or so they thought.

Viktoriya has faced her fair share of demons. Literally. After Viktoriya's sister is murdered, she makes a vow to strike back, hoping to put as big a dent in Hell's population as possible.