The Book: The legend of John and Anna

"Every millennium two leapians are born to different families, a boy and a girl, when they are of age the book appears, no one knows where it came from, no one knows what it does and no one can understand it except the two leapians, as soon as the book appears everyone disappears, everyone, children, old people, even animals and the only two people in the world are the leapians and that's when the demons start to appear, just like the book no one knows were they came from but when they arrive they come after the book and the leapians, no one knows why, all I know is we're supposed to read the book and figure out what it wants us to do to get everyone back and kill the demons."