eidolon

A dark wisp of smoke—Percy guessed it must be an eidolon—seeped into a Cyclops, made the monster hit himself in the face, then drifted off to possess another victim.

They gathered around the dining table, and Piper explained what had happened at Topeka 32—their conversation with Bacchus, the trap sprung by Gaea, the eidolons that had possessed the boys.

Helen doesn’t ask anyone at Prime Space to explain her mission’s name, Eidolon, but if she’d checked Wikipedia, she would have learned that an eidolon is a phantom in human form.

“The eidolons—the possessing spirits. They’re still here, in this room.”

These eidolons acted a lot like the spirits in her dad’s stories.