The Tyrant's Tomb by Rick Riordan
It’s not easy being Apollo, especially when you’ve been turned into a human and banished from Olympus. On his path to restoring five ancient oracles and reclaiming his godly powers, Apollo (aka Lester Papadopoulos) has faced both triumphs and tragedies. Now his journey takes him to Camp Jupiter in the San Francisco Bay Area, where the Roman demigods are preparing for a desperate last stand against the evil Triumvirate of Roman emperors. Hazel, Reyna, Frank, Tyson, Ella, and many other old friends will need Apollo’s aid to survive the onslaught. Unfortunately, the answer to their salvation lies in the forgotten tomb of a Roman ruler . . . someone even worse than the emperors Apollo has already faced.