Percy Jackson is slowly losing his mind, but the others expect him to lead them. So he does, pretending that he wasn't affected by Annabeth's death, his parents death, and being tortured in Tartarus by all who wanted revenge against him; just like any other hero would.
But the Gods couldn't be fooled. They noticed how their young hero looked when he thought no one was watching, and they noticed how the nightmares affected him.
So when a group called the Avengers seek help from the Gods in a new war, who will they send to help the Avengers?