The Letter of Conspiracy

Harry Potter isn't sure he wants to be a Slytherin. Nevertheless, he is, and whilst trying to survive the snake pit he notices that things aren't quite right in the Wizarding world. History is being rewritten, and sinister forces are looking to change magic itself. But can he win the Game when he doesn't know all the players and a Dark Lord is trying to kill him? Dark!Slytherin!AU!