The Heroes of Olympus series is the story of a fight between good and evil. The heroes—Percy, Annabeth, Jason, Piper, Frank, Hazel, and Leo—are the good guys trying to stop the forces of evil, Gaea and her monstrous army, from overthrowing the world. The House of Hades explores the difference between good and evil.

The heroes sometimes do bad things, but what separates them from villains is their recognition of the mistake and how they go about making amends. For example, Percy has been the hero since the Percy Jackson books. He has a good heart and tries to do what is best, but when he’s traveling through Tartarus, he realizes that he has done some bad things and does not deserve Bob’s help. When he is fighting the arai and gets curses from all the monsters he’s fought, Bob deserts Percy because Percy manipulated him. This causes Percy to face the full weight of his mistakes:

He couldn’t blame anyone else for his troubles. Not the gods. Not Bob. He couldn’t even blame Calypso, the girl he’d left alone on that island. Maybe she’d turned bitter and cursed Percy’s girlfriend out of despair. Still…Percy should have followed up with Calypso, made sure the gods sprang her from her exile on Ogygia like they’d promised.