Harry Potter and the Goblet of Fire
Under the panic caused by his arrival, Moody takes a traumatised Harry to his office. He reveals himself to have been working with Voldemort the whole time as a 'faithful servant'. He explains that he put Harry's name into the Goblet of Fire and has been guiding him through the tournament to ensure he would touch the Cup first. As Moody prepares to kill Harry, Professors Dumbledore, McGonagall, and Snape intervene and stun Moody. Slowly, Moody changes appearance to become Barty Crouch Jr., the supposedly long-dead son of Barty Crouch Sr., who was using Polyjuice Potion to impersonate Moody.