The Underworld (Fallen Star, #2)

(Same edition, new cover)
The Underworld is the sequel to The Fallen Star (Fallen Star Series, Volume 1)
Gemma thought her mind was gone, but she was wrong. And now she is left trying to figure out the truth to what Stephan is planning to do with her and the star, before it’s too late.
But finding out the truth is hard, especially since Gemma doesn’t know who she can trust. There may be only one person who Gemma can turn to for answers, but that means having to go to the one place no one wants to go—The Underworld.