After the Battle (Percy Jackson)

AU where Percy Jackson dies after the battle with Gaea.
Annabeth's a wreck, as is almost every demigod, Roman or Greek. Nico is MIA, Jason and Frank blame themselves, and Blackjack won't even eat.

But something is stirring; a new evil is rising.
In the Underworld, Percy is offered something miraculous from the Gods, that no one has ever been offered before: a chance to come back, temporarily. A chance to save his friends one last time.
The catch: they can't know he was ever there.