Book 3 of Twins of Death and Sea.
The gang while recruiting two new demigods bumps into the Hunters of Artemis as well as Artemis herself. When they defeated the monster, Annabeth falls down a cliff and gets captured by the titan Atlas who has captured Artemis as well.
Zoë Nightshade, the lieutenant to Artemis, after finding out that Artemis has been taken, makes the decision to go on a quest to find her. At first she brings Grover Underwood, Bianca Di Angelo, and Thalia Grace, but takes Percy Jackson with them after they defeat a monster. Now they all have to go and save Artemis and Annabeth before the winter solstice comes and goes.
While they are doing this, Heather has a quest of her own. She has to go to the place that never sleeps to find the land of the unbounded. The purpose? She doesn't know. She just wants to get there first and plan after, especially since it said she'll destroy the world. She has to figure out what the prophecy means and how to accomplish it before it's too late.