Humanity needs literature. Without literature, human civilization will end. The Taint wants to destroy all the human literature on the planet. They do this by entering a book, subsequently sucking the author into the book. The Taint then tries to change the ending of the story. However, if the author manages to finish the story as it was written, then the book will be saved. And the author will subsequently get magic powers and be placed on the front-line of defending books.