The DemiZard Saga: The Maze of Magic.

Well, in the Giant war we've seen that Percy and Jason are exchanged, between the Roman and Greek Demigod camps respectively.
After the Second Wizarding War, Hogwarts offers Harry and other last year wizards and witches to study the seventh year and complete their studies. Harry thinks that everything is over. Now he can live a happy and not-very-harmful- adventurous life.
After the Giant War, Percy thinks that life should be simple now. He wanted to spend as much as time with Annabeth and Rachel, he wanted to play Capture the Flag, he wanted to play volleyball with the campers, to roam the forest, to talk with Chiron, to visit his mom every Sundays, visit Tyson and his father at his kingdom every month and make love with Annabeth. He wanted to be just a simple boy, not an always-in-danger-demigod.

But the Fates had just different plans for both of them. As if the Fates have decided not to let the boys live happily.
While the Giant War was being fought, the doors of death were open, which unleashed the most powerful and darkest villain of all time.