The Guardian - Sherrilyn Kenyon

As a Dream-Hunter, Lydia has been charged with the most sacred and dangerous of missions. She's to descend into the Nether Realm and find the missing god of dreams before he betrays secrets that could kill all of them. What she never expects is to be taken prisoner by the Realm's most vicious guardian.Seth's time is running out. If he can't hand over the entrance to Olympus, then his own life and those of his people will be forfeit. No matter the torture, he hasn't been able to break the god in his custody. But when a rescuer appears, he decides to try a new tactic. If he can win her to their cause, then he can save all their lives.When these two lock wills, one of them must give. Lydia isn't just guarding the gates of Olympus, she's holding back the darkest of powers from the earth itself. If she fails, an ancient evil will roam the earth once more and no one will be safe. But evil is always seductive . . .