This isn't a story, per se, but just a crazy realization that I had that I'm not sure if other people picked up on.
What if Kronos, in The Last Olympian, is just an Eidolon?
Think about it, an eidolon is a ghost who possesses the living. Kronos' body was chopped into pieces, destroying his body, so he needed a new one, so his ghost possessed Luke.
And what color did Percy and Jason's eyes turn when they were possessed? Gold. Who else in the series has gold eyes? Luke, when he's possessed by Kronos.
Now, Kronos wasn't trying to be subtle like our ghosty friends on the Argo II, so maybe eidolons can keep the possession up for a really long time, maybe indefinitely. I don't really know about this part, maybe Kronos is just a really powerful one because... He's Kronos, an actual Titan.
Now, I don't really have any more evidence of this at this exact moment, but if you have any additions, let me know and I'll add them! Or if you think I'm wrong. then we can start a healthy debate!
If you have any more theories you want me to cover, let me know and I'll look into them!