This is a Percy Jackson and harry potter crossover. Percy Jackson receives a quest from Dumbledore to go protect Harry at Hogwarts I know that the time periods for each separate character doesn't make sense, so let's ignore it.

This is the story of an extraordinary little girl. Orphaned as a baby, Emily grows up as a peculiar ten-year-old with abilities that not only frightens the other children but also herself. Read along as she lives her life as the now adopted daughter of a powerful woman, a professor of Hogwarts, A school of Witchcraft and Wizardry. The time has come for her to know of her family and of her otherworldly legacy as Emily Potter, the girl who lived. - Edited as of July 2020 - Photo credits: http://vignette2.wikia.nocookie.net/camphalfbloodroleplay/images/5/5e/Alone-black-hair-girl-sad-Favim.com-428528.jpg/revision/latest?cb=20120601103952

Percy Jackson is broken. After the Giant War, Camp Half-Blood's protective barriers were destroyed, and he was the only one who escaped. He watched his friends and family be slaughtered by an army of monsters, and he has nothing left to lose. When Professor McGonagall comes to ask if he could teach a class called Defense Against the Dark Arts in a school called Hogwarts, he agrees. But there was one thing that will be difficult- not revealing that he is a demigod. Will he keep the secret, or will the year end in disaster? I do not own the Percy Jackson series, Harry Potter series or the characters in them. They are owned by Rick Riordan and J.K. Rowling.