At the start of a game of Chess, choose one piece from the opponent's Team. This piece is now your Spy. At any point in the game, but only during your turn, you can declare "Spy turn" and your opponent must close her eyes. You may then move this piece.
At any point during the game, you may declare "A spy is among us!!!" and nominate which piece of your team you believe to be a spy. Your opponent will inform you if you are correct. If you are correct, you may eat the piece.