The Battle

What if, after the Battle of Hogwarts, Harry, Hermione and Ron were put into a cross-dimensional apprenticeship program? What if Percy Jackson and Annabeth Chase were there too? What if every fictional world you've ever read was real?

What if, by crossing over two worlds, another was destroyed?

Recommended to read before this:
Percy Jackson and the Olympians (by Rick Riordan)
Heroes of Olympus (see above)
Harry Potter (by J. K. Rowling)
The Inheritance Cycle (by Christopher Paolini)
Adventures Wanted (by M. L. Forman)