The seven Demigods are in fact shaken after their last adventure with the Wizarding World. However, since then, Annabeth has been concerned as to what other worlds are out there, and decides to start traveling around the world and exploring, taking the others with her. Stumbling upon a high mountain, Annabeth and the others fall through it, and into an Underground world of magical creatures. As they try to find a way out of this world, they meet and gain new friends along the way. Little do they know that a dark soul has awakened...and he will do everything in his power to make sure that they have a bad time. (THIS CROSSOVER WAS MADE AS A SEQUEL TO THE GAME UNDERTALE, BY TOBY FOX. ALL CREDITS GO TO HIM FOR MAKING THE GAME, AS WELL AS THEY GO TO RICK RIORDAN FOR MAKING THE HEROES OF OLYMPUS. ALL I OWN IS THE ALTERNATE CROSSOVER STORYLINE :D)

well, to start off my crossover series, I'm going to make a crossover with Heroes of Olympus and Harry Potter. It takes place after all of the "Heroes of Olympus" books in the series, in which everyone is having a casual day at Camp Half-Blood, and Leo has gotten back to Earth with Calypso safely by his side... UNTIL Percy and the others accidentally get involved with Harry's Potter's final quest to destroy the dark lord, Voldemort! Forming an alliance, Percy Jackson and the rest of Camp Half-blood, work together with Harry Potter and the students of Hogwarts to protect everything they hold dear to their hearts..., and defeat the forces of darkness! I DO NOT OWN HARRY POTTER, HEROES OF OLYMPUS, OR ANY OF THEIR CHARACTERS, AND ORIGINAL STORYLINE. ( HOWEVER, THIS CROSSOVER STORYLINE WAS MADE BY ME :D )