Cass and Erin are children of light and darkness. They will need both to rescue the Furies and make it out of the Underworld before it's too late.

Finding out he was part Fury was weird enough, but now Cass is thrown into a battle he's not sure he wants to fight. He just wants to go back to his normal life, but the gods (and his sister, Erin) have other plans.

With the Furies imprisoned in Tartarus, Apollo and the goddess of nightmares are busy gathering the pieces for their war against Olympus. To stop them, Cass and Erin must harness all of their demi-Fury powers to go to the Underworld and bring the Furies home.

It's not going to be easy. Saving the Furies means they'll need to stay one step ahead of the goddess of nightmares, break into the dungeon of Tartarus, and battle the monsters who are gathering in the Underworld.

They have three days to outsmart the gods, unite their power, and get their family back before they're trapped in the Underworld forever.