Cavern

Finally the ghost with the pope hat struck his shepherd's crook against the floor. The other Lares fell silent.
"Your Greek goddess is powerless here," said the pope. "Mithras is the god of Roman warriors! He is the god of the legion, the god of the empire!"
"He wasn't even Roman," Annabeth protested. "Wasn't he, like, Persian or something?"
"Sacrilege!" the old man yelped, banging his staff on the floor a few more times. "Mithras protects us! I am the pater of this brotherhood — "
"The father," Annabeth translated.
"Do not interrupt! As pater, I must protect our mysteries."
"What mysteries?" Annabeth asked. "A dozen dead guys in togas sitting around in a cave?"
The ghosts muttered and complained, until the pater got them under control with a taxicab whistle. The old guy had a good set of lungs. "You are clearly an unbeliever. Like the others, you must die."
The others. Annabeth made an effort not to look at the skeletons.
Her mind worked furiously, grasping for anything she knew about Mithras. He had a secret cult for warriors. He was popular in the legion. He was one of the gods who'd supplanted Athena as a war deity. Aphrodite had mentioned him during their teatime chat in Charleston. Aside from that, Annabeth had no idea. Mithras just wasn't one of the gods they talked about at Camp Half-Blood. She doubted the ghosts would wait while she whipped out Daedalus's laptop and did a search.
She scanned the floor mosaic — seven pictures in a row. She studied the ghosts and noticed all of them wore some sort of badge on their toga — a raven, or a torch, or a bow.
"You have rites of passage," she blurted out. "Seven levels of membership. And the top level is the pater

The ghosts let out a collective gasp.