Barty Crouch Junior
Background
Bartemius "Barty" Crouch Junior is a former Death Eater and follower of Voldemort, currently imprisoned in Azkaban. His father is Barty Crouch.
During the events of Harry Potter and the Goblet of Fire (2005) played by David Tennant
Frank Bryce hears Voldemort speaking to Peter Pettigrew. Frank looks into a room and sees Peter speaking to what appears to be a tiny figure in a chair. Voldemort tells Barty Crouch Junior to gather their old allies. Nagini slithers into the room and tells Voldemort that Frank is listening outside, and Voldemort uses a spell to kill Frank. After the Death Eaters attack the camp where the Quidditch Championship is being played, Barty Crouch Junior walks around the camp after it has been destroyed and makes the sign of the Dark Mark in the sky. Barty Crouch Junior, disguised as Alastor "Mad-Eye" Moody, walks into the Hogwarts hall and is greeted by Albus Dumbledore.
In class, Moody introduces himself as the new Defense Against the Dark Arts teacher. He teaches the students about the Unforgivable Curses by using them on a tailless whip scorpion, finishing the class by using Avada Kedavra to kill the tailless whip scorpion. Draco Malfoy is about to cast a spell on Harry Potter when Harry's back is turned when Moody sees him and transforms Draco into a ferret. Professor McGonagall turns Draco back from a ferret and then chastises Moody. Moody takes Harry into his office and works out a strategy to defeat the first challenge. As Barty Crouch is congratulating Harry for placing second place in the second challenge, Moody walks up and Barty notices a mouth tick that Moody exhibits and seems to recognize it. Junior later kills his father.
The final challenge begins and the challengers enter a maze. Harry is transferred back to the maze location and Dumbledore notices something is wrong. Harry yells that Voldemort is back and Moody pulls Harry away while Amos cries over Cedric Diggory's body. Moody asks Harry how it felt to stand in front of Voldemort. Moody starts to transform and as he is about to kill Harry, Dumbledore bursts into the room and stuns Moody with magic. Professor Snape pours a potion into Moody's mouth. Dumbledore, Snape and McGonagall find the real Moody locked in a chest and the fake Moody fully transforms back to Barty Crouch Junior.