|“||The Earth Mother sent three. The strongest, the best. We will live again.||”|
–The Eidolons, in The Mark of Athena
Eidolons are spirits that first appear in The Mark of Athena. They are shown to have the ability to possess people and machines. They serve the earth goddess Gaea in The Heroes of Olympus series, with the reward of returning to the living world again.
Shortly after the Argo II left Camp Half-Blood to try and make peace with Camp Jupiter, three eidolons sent by Gaea appeared on board. They seemed to taunt Annabeth Chase for most of the trip, giving her a cold feeling whenever they were near and seemed to laugh in her ear, making it seem like every decision she made was the wrong one. After the Argo II stopped and hovered above new Rome, Reyna invited the demigods to a meal, which they accepted as they talked about the Prophecy of Seven. Octavian protested the meeting and thought that the war ship would fire on them. Leo Valdez offered to show him the ship, which Reyna said was a good idea and sent Octavian off. Just at that moment, an eidolon took control of Leo and Annabeth saw him give an evil smirk that only lasted a second, making her think she was seeing things.
Once on board the ship, Leo began loading the ballista and firing on Camp Jupiter, leading to a fight between the Roman and Greek demigods below. Once on board the ship, Leo was knocked out by Percy Jackson. Once the Argo II sailed far enough away from Camp Jupiter, Leo commented that while he could see himself attacking the camp, he couldn't control himself and felt a kind of cold feeling as it happened. While the others decide that Leo must have been controlled by some kind of magic, they feel that he can't be left alone anymore in case it should happen again.
Later, while Piper McLean, Percy, and Jason Grace are meeting Bacchus in hopes of getting information, Bacchus feels that it is a trap and vanishes. At that moment, Percy and Jason are possessed by two more eidolons and begin to fight each other, saying that one must die. While Percy manages to knock Jason out, Percy is also knocked out by Blackjack and flown back to the ship.
During a meeting on the ship a short time later to talk about what they learned from Bacchus, Piper mentions that the eidolons are still on the ship and she has a plan to get them off. She uses her Charmspeak to call them out, which Leo finds funny until he notices his hand go up as Piper instructed, once again being taken over by the eidolon. The eidolons inside Jason and Percy are also called out. They admit that Gaea sent her three strongest to go after them and would be allowed to keep whatever bodies they wanted if they succeeded in their mission. Piper forces them to promise on the River Styx to never return to the Argo II or possess any member of the crew, which they unwillingly accept.
However, the eidolons wouldn't give up, they found a loophole in what Piper said. They followed the crew all the way to Rome and took control of three tourists to follow Leo, Hazel, and Frank to an old monument. When Leo uses his abilities to find an ancient lock, Hazel goes into the tunnel to see if it is safe while the three eidolons watch Leo and Frank. They eventually make their move, forcing Leo and Frank to follow Hazel into the tunnel. Once inside, they find an ancient work shop full of sphere's made by Archimedes. The three eidolons posses some of the machinery and use an electric sphere to knock out Hazel and Frank. It is then revealed that one of the eidolons possessed Leo to fire upon New Rome using the ballistae of the Argo II and was in his head, so he knows how Leo thinks. Leo is forced to ask Nemesis for help, allowing him to access a sphere that controls the other spheres. He takes control of the eidolons' machine bodies and overloads them, trapping them in the machinery's wiring and then melting them, making it so they can't escape.
While eidolons don't properly appear, very close to the Doors of Death, Percy spots a wisp of smoke which he assumes to be an eidolons. It takes control of a Cyclopes, punches itself and leaves the body to find another host. The book did not say that the Cyclops had a golden eye.
It is unknown what the eidolons look like, as they possess other people and objects. However when they take control of someone, they all share the same golden eyes, and when they speak while possessing someone, they "had a hollow voice, and he spoke haltingly, like English was a second language" as described in the The Mark of Athena. However, while locating the Tartarus side of the Doors of Death, Percy described an eidolon as a "dark wisp of smoke".
- Possession: Eidolons are able to take control of other people or machines. When taking control of a person, the possessed person is able to see what is happening, but is unable to do anything about it. While possessing someone, they learn everything the person knows, including things like sword play or how a person thinks. Possessing a person doesn't cause the person harm unless the eidolon makes them hurt themselves.
- Camouflage: Eidolons seem to be able to hide their presence from those they are possessing, meaning that those being possessed will be unable to tell what actions were theirs and what actions were caused by the eidolons.
- Frank correctly figures out that while Piper made them promise not to come on board or possess any member of the Argo II, she didn't make them promise they couldn't follow or possess new bodies, leading Leo to comment "Eidolons who are also lawyers."
- Eidolons may be warded away by charmspeak.