Sentinel Book by Jennifer L. Armentrout

It’s a beautiful day for a war. As the mortal world slowly slips into chaos of the godly kind, Alexandria Andros must overcome a stunning defeat that has left her shaken and in doubt of their ability to end this war once and for all. And with all the obstacles between Alex and her happily-ever-after with the swoonworthy Aiden St. Delphi, they must now trust a deadly foe as they travel deep into the Underworld to release one of the most dangerous gods of all time. In the stunning, action-packed climax to the bestselling Covenant Series, Alex must face a terrible choice: the destruction of everything and everyone she holds dear… or the end of herself.