So you've got this mystery, such as the identity of the villain. There are a bunch of theories. Then The Reveal. Wait, who's that? You have just met the Stranger Behind The Mask, where The Reveal proves to be something or someone we've never heard of before, and had no way of expecting. This can often result in an Anticlimax, and is almost always a Shocking Swerve. Both Ronald Knox and S.S. Van Dine attempted to create rules for Detective Fiction, one of which was created in order to either prevent or avert this trope from occurring. Knox, indeed, made it his first commandment: "The criminal must be someone mentioned in the early part of the story..."