Percy Jackson, his friends, and Harry Potter and his friends, go for a hike on Mt. Ebott, when they all disappear. They end up in the Underworld and need to escape, but decided to help the monsters escape as well. Either they sacrifice one of themselves or find the hidden object that connects the worlds. (Btw this is a crossover between 3 Fandoms, PJO, Harry Potter, and Undertale)