The Butt-Kicking Sons of Chaos (Percico)

Percy Jackson is overjoyed when his half-brother turns up at camp. At least, until his brother turns out to be a massive jerk. He stole Percy's girlfriend, Annabi-Annabeth AND turned the entire camp AND Olympus against him! Except one person. Nico di Angelo. Yeah, Nico. The creepy kid who seems to have a personal vendetta against Percy. Well, he does-sort of. It's a long story.
The two unfortunately die, when they meet Chaos, creator of the universe. Apparently, Chaos decided he liked them so much he adopted them and made them commanders of his army! In the process of commandering for 10 thousand years, the two develop some very healthy crushes, and to us fangirls' delight, get together as a couple!
The Fates aren't on their side, though. Chaos got called to Olympus to clean up the gods' little mess. Gaia AND Kronos AND Chaos' sister Order are loose in America, and Chaos' army has been summoned to help deal with them. and guess where the army'll be staying? In CHB, of course! Aside from fighting the bad guys' army, Percy and Nico also have to deal with nosy campers *cough* Annabeth *cough* trying to look under their hoods, stalking them, and generally not minding their own business.
It's complicated. Just like everything related to demigods.