Fatal Ties — J.A. Cipriano

Lillim Callina is good at fighting monsters, but when she finds out Nidhogg, the ancient dragon confined beneath the world tree, is breaking free, well, let's just say if she can't stop him, Ragnarok is a thing. Fortunately, she'll have help. Connor, a demi-god possessed by an Ancient Evil destined to destroy the world has agreed to help her stave off the apocalypse, which sounds all well and good if you can ignore the whole “demi-god possessed by an Ancient Evil” thing. So yeah, probably not the world's most trustworthy ally, but any port in a storm, right?