The immortal fire (Atheneum)

After their near-fatal battle with Poseidon, Charlotte and Zee would love nothing more than to relax and forget all over again that the Greek gods are real. But with the world in peril and no one else to save it, that just isn't an option. Charlotte and Zee meet the Prometheans, an ancient brotherhood trying to protect mankind from the whims of the gods, who think they might have finally found a weapon capable of bringing Zeus to his knees. But using this weapon will come at a great cost, one Charlotte and Zee are not willing to pay. They strike out on their own for Mount Olympus, with the Prometheans, an angry Chimera, and all sorts of mythological beings on their tail...