The Court of the Dead

It's been three months since demigod boyfriends Nico di Angelo and Will Solace returned from Tartarus, and there hasn't been a single quest for them in all that time. So, when Nico's half sister Hazel asks them to come to Camp Jupiter to help with an 'issue', Nico shadow-jumps at the chance to do something.
What Hazel didn't tell the boys is that she's providing a haven for a group of monsters who've escaped the Underworld! Apparently, during Nico and Will's trip to the Underworld, they showed the monsters that they don't have to be evil - that the monsters have options.
But then the monsters start disappearing from the camp and it's clear a dark force is at work - and Nico, Will and Hazel are shocked to find that they're connected to it . . .