Beautiful Chaos (Book 3)

When he looks at Lena, it's like there's no one else in the world. But Ethan is mortal, and on her seventeenth birthday Lena made a choice that changed everything. The girl Ethan loves has broken the world with the supernatural powers she is struggling to control ... Now, if they are to fix the chaos Lena has caused, one of them must make a terrible sacrifice. Sometimes there isn't just one answer. Sometimes there's no going back. And this time there won't be a happy ending.

Kami Garcia and Margaret Stohl