Percy Jackson at Hogwarts
Percy Jackson, a demigod well known. He just came back from his battle with Gaia, still shaken up from the experience. One day, he came back to camp only to see Chiron asking him to go to a quest. Chiron wants to know if the wizards can be allies, but one problem. Voldemort is still alive. He came back after escaping death again, tricking the God Hades, and in the process getting the God angry. Percy needs to go to Hogwarts, a school for witchcraft and wizardry.
Harry Potter, killed Voldemort. He finished his last year in Hogwarts, but was asked to retake the year, since the experience affected their education. When he gets to school he meets this weird exchange student. He excels in all classes, and is friends with almost everyone. Little did Harry know that he is here to defend Hogwarts from his old enemy, Voldemort.
Disclaimer: I do not own the characters, all rights go to Rick Riordan and J.K. Rowling, but the plot is mine.