Amara's Rose, Book Two: The Fall:

Almost a year after Rose returns to Amara a threat greater than any she has ever faced looms on the horizon. Determined to forsake a prophecy predicting the end of the worlds, Rose embarks on a mission to uncover the secrets of an ancient being known as the Darkness, all while ignoring a vital piece of the foretelling that includes Derek.
Abandoned on Earth with no working Key, Derek tries to forget that there are other worlds out there to be discovered and the blonde Amarian who won't leave...