The Guardian of the Underworld

Who's messing with the power switch on the human race? Is Frank getting too old to fight? Is Nico being possessed? It's been almost 10 years since the destruction of Mother Gaea and Porphyrion, and things were finally starting to go back to normal. But just like their whole lives, something has to come and try to sabotage the world. Can the save themselves and everyone else, or will they have to settle down and wait for the apocalypse to start?
DISCLAIMER: I don't own anything but the plot and the few new characters! Rick Riordan own everything else!
Also, this storyline happens years later, but doesn't include anything that happens from the series after the Heroes Of Olympus.