Apollyon (Covenant, #4) by Jennifer L. Armentrout

"I heard him whisper…You are my everything."

Everything Alex has feared for the past few months has come true: She has lost her free will to the Awakening, and has been placed on the Elixir, which makes her docile and compliant. She has become a shell of herself. But love has always been stronger than fate, and Aiden St. Delphi is willing to wage war on the gods—and Alex herself—to bring her back.

The gods have killed thousands and leveled cities in their quest to end the all-powerful bond between the Apollyons. They would do anything to stop Seth from becoming the God Killer. But breaking Alex's connection to Seth isn't good enough anymore, because Seth has become ruthless in his hunger for power. There are a few pesky little loopholes in the "an Apollyon can't be killed" theory, and the only person who might know how to stop the destruction has been dead for centuries.

Alex and Aidan must venture their way past the guards of the Underworld and search for one soul among countless millions. As if that wasn't already hard enough, Alex has been desperately trying to keep Seth at bay from becoming the God Killer, but what she never realized is that she could become the God Killer herself.