Barty Crouch, Jr
Bartemius "Barty" Crouch, Junior was captured with the Lestranges, who tortured Frank and Alice Longbottom, parents of Neville Longbottom into insanity. His father, Barty Crouch, Sr., who headed the Department of Magical Law Enforcement at the time, sentenced him to life imprisonment in Azkaban. However, he later rescues his son as a favour to his dying wife. When they visit him in Azkaban, Mrs. Crouch used Polyjuice Potion to switch appearances with her son, enabling him to escape while she remains in his place. When she dies, she is buried under his identity. Crouch Jr is nursed back to health by Winky, the family's house-elf.
To prevent him from returning to Voldemort's service, Crouch Sr controls his son with the Imperius Curse and keeps him hidden under an invisibility cloak. When Bertha Jorkins discovers the truth, the news reaches Voldemort, who rescues Crouch Jr and puts Crouch Sr under the Imperius Curse. Crouch Jr then imprisons Mad-Eye Moody, a famous auror, and using Polyjuice Potion assumes Moody's appearance and position to infiltrate Hogwarts as the new Defence Against the Dark Arts teacher. Despite not being a real teacher, Crouch does a fair job and the class learns vast amounts from him. Crouch Sr escapes and, exhausted and delirious from the Imperius Curse, reaches Hogwarts to tell Dumbledore about Voldemort's return; but his loyal Death-Eater son murders him on the grounds.
The revived Triwizard Tournament is held at Hogwarts, and Voldemort tasks Crouch Jr with making sure Harry wins. To do so, he bewitches Victor Krum to attack Cedric Diggory in the maze and stunned Fleur Delacour. When Harry and Cedric simultaneously touch the Triwizard Cup, which is a portkey, it transports them to a graveyard. There, Death Eater Peter Pettigrew uses Harry's blood in a ritual that brings Voldemort back to power. The plan succeeds, but Harry escapes. When Harry returns to Hogwarts, the still-disguised Crouch Jr hopes to succeed where his master failed; but Dumbledore, Snape, and McGonagall foil his plot. Under the effects of Veritaserum, he recounts his plan to them. Although he is closely guarded so he can later repeat his testimony, Minister of Magic Cornelius Fudge has a Dementor suck out Crouch's soul in order to silence him before he can be brought to trial and confirm Voldemort's return. He lives bereft of his memories or sense of self.
In the film he is played by David Tennant
Read more about this topic: Dolohov, Notable Death Eaters
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