Leo comes back to Camp Half-Blood 4 months later, without Calypso. She ran off with some minor God and never loved Leo. The seven were on a mission and Leo gets a fatal hit. Will he survive?
When the gods do not keep their promise and Calypso dies in Leo's arms for leaving the island, Leo comes back to CHB a changed person. He doesn't crack jokes anymore, he ignores his friends and isolates himself from everyone.
But most importantly, he wants to take revenge on the gods, and no one was going to get in his way.