Son Of Neptune (Heroes Of Olympus Book 2) by Rick Riordan
One cursed demigod. Two new heroes. A quest to unleash the god of death . . . I’m the God of Rome. I protect the legions. I don’t want war without end. You will serve me.’ ‘Not likely,’ Percy said. ‘I order a quest!’ the god announced. ‘You will go north and find Thanatos in the land beyond the gods. You will free him and thwart the plans of the giants.