Hades - Guardians of Hades Paranormal Romance Series, #9

The fate of our world rests in their hands. Hades, god-king of the Underworld, has uncovered the mastermind behind the uprising destined to set his realm and the mortal world on a collision course. But his enemy has gone to ground and finding them is proving impossible, and the longer they elude his grasp, the stronger the darkness within him grows, slowly stealing control. Persephone, god-queen of the Underworld, can only watch as a storm gathers on the horizon-a deadly war that threatens to strip her beloved family from her.
As Hades fights his own battles against their enemy, she fights to keep the darkness within him at bay, never more aware of the danger he would be if she failed to keep it tamed. But when their enemy strikes out at them from the shadows, dealing a blow that changes the course of the war and incites Hades's wrath, the leash on the monster within him snaps and he is plunged into the darkness. And saving this world might no longer be an option.