The Guardian - Sherrilyn Kenyon

Dream-Hunter Lydia has been charged with the most sacred and dangerous of missions: To descend into the Nether Realm and find the missing god of dreams before he betrays the secrets that could kill her and her kind. 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, his own life and those of his people will be forfeit. No matter the torture, Seth hasn't been able to break the god in his custody. Then there's the beautiful Dream-Hunter Lydia: She isn't just guarding the gates of Olympus--she's holding back one of the world's darkest powers. If she fails, an ancient curse will haunt the earth once more and no one will be safe. But evil is always seductive...