Harry Potter is one of those 'once in a generation' type of stories. Perhaps more so than any work of children's literature of the last century, the Boy Who Lived has influenced the lives of billions across the world. In the 21 years since the first book was published, J. K. Rowling's wizarding world has spawned books, films, theme parks, some truly terrible fan-fiction, and even a genre of Harry Potter inspired metal music called 'Dumblecore'.