The House of Hades by Rick Riordan
Leo, Hazel, Jason, Piper, Frank, and Nico are sailing the Argo II, trying to reach the Doors of Death in Epirus, Greece. Meanwhile, Percy and Annabeth are struggling through Tartarus, trying to get to the Doors of Death on their side. The crew on the Argo II have many challenges to face, new enemies and old. Hazel runs into the goddess Hecate, but Hecate might be fighting for Gaea. Nico and Jason face Cupid, leaving one of Nico's biggest secrets in the open. Frank takes a step in the direction of his father, changing him for the better or worse. Leo, Jason, and Piper face an old enemy, which leads to Leo facing a harder challenge than he ever imagined. Camp Jupiter is ready for war against Camp Half-Blood, and the seven can't do anything to stop it without sending someone back to the camp. Old friends make an appearance, including Rachel Elizabeth Dare and the love-able Grover Underwood. Octavian and Reyna also are back, and Octavian is an ann...