Eidolons are possesing spirits, whose goal are to live again in the body of another.
The Mark of AthenaEdit
An eidolon posseses Leo Valdez to attack Camp Jupiter, on Gaea's orders, causing Rome to count Camp Half-Blood as one of its enemies. This was exactly what Gaea wanted to happen. Later, Gaea eidolon-possesses Jason Grace and Percy Jackson into a battle, which would have been to the death, if not for Piper McLean's charm-speaking abilities. The eidolons are permanently trapped inside of melted automatons at the end of the book.