Desolation Series by Ali Cross

The battle over Midgard begins with just one girl ... Earth has been without a Guardian since its creation, but Loki means to take it for himself. His daughter, sixteen-year-old Desolation, wants nothing more than to stay in Hel where it's cold and lonely and totally predictable. Instead, she's sent to Midgard to make her choice--and what she chooses will determine not only her own future but the fate of all the worlds.

Where darkness lives, all will become desolate. Desi chose Midgard over Hel, over her father, Loki, but she couldn't banish the Shadow that lives within her. Working with The Hallowed, she remains vigilant against her father's forces. When she's injured by a strange demon, old temptations rise, and the darkness that hides within her may mean the destruction of all that she's been fighting for.

The fate of Midgard will be decided. The goddess of Helheimer is bent on reclaiming what once was hers. The nine worlds are drawn into war, over Hell, over Earth, with innocent humans as collateral. When all seems lost, Desi discovers the meaning of the spark that lives within her and is offered her final choice: eternal peace or the weight of Thor's Hammer in the battle for Midgard.