Kara has lost her elemental power, and with her guardian angel days behind her, all is back to normal—or so she thought. But when she's injected with a mysterious substance by a shadowy man, she begins to change...but into what? As Kara deals with the change that brews inside her, a new menace threatens the angels and the mortal world: Reapers.

And now back in Horizon, it's up to Kara and her friends to figure out who is powerful enough to control the Reapers and how to stop them. What's worse, rumors of something far worse than Reapers surface, of beings more sinister and evil. Something that the Legion believed they had banished forever is now resurfacing—creatures rumored to be so powerful they could destroy Horizon forever.