After Hazel Levesque relives some of her memories from her old life, Percy Jackson anchors the Pax onto a nearby shore. While Percy went back to get supplies from the boat, Frank Zhang notices that the field of grass was still moving, even after the wind had stopped. A moment later, Frank was knocked back and Hazel was dragged across the field in a funnel cloud of grass. While trying to find anything to grab to stop herself, Hazel uses her powers to pull a large rock from the ground called "schist." Three karpoi appear at the base of the rock and complain about the schist as they couldn't grow on it. The three introduced themselves as Wheat, Barley, and Sorghum and said they captured Hazel in order to turn her into the Giant Army.
The three Karpoi then offer to give Hazel their elements in exchange for Hazel turning herself in to the enemy army. She agrees to in order to give Frank and Percy time to find her, she then gets the spirits to argue among themselves over which of them is better. She also asks why they don't serve Ceres, Goddess of Agriculture, but they explain that they hate her because they are wild and Ceres taught people how to cultivate them. Hazel also tried to find out the location of Thanatos, but Wheat saw through her plan and stopped the others from telling her.
They then went to attack her, but Percy and Frank appear from the field and destroy the spirits. A moment later they try to reform as other monsters had done, but Percy claimed he would burn the entire field if they didn't get lost, with Hazel backing him up claiming he would do it because he was crazy. The karpoi then run away out of the field. They are later seen along with the rest of the Giant Army when they attack Camp Jupiter.
Meg summons a Karpoi, Peaches, to aid her through the book.
He is saved from being burned by Meg.
They are the size of toddlers, with rolls of baby fat, but their skin has a greenish hue from the chlorophyll in their veins. They have dry, brittle wings like corn-husks, and tufts of white hair like corn silk. Their faces are haggard, pitted with kernels of grain. Their eyes are solid green, and they have canine fangs as teeth. They can also dissolve into whirlwinds of their chosen grain. They also explode into various cereals like Weet-Bix and Milo.
- All the Karpoi are named after different types of grain, and fruit.
- They hate Ceres because she cultivates them.
- When they die, they turn into various grains and cereal, such as Chex Mix.