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Voldemort
He-who-must-not-be-named or Voldemort is the leader of all forces Evil in Harry Potter. 
Voldemort is the Harry Potter's archenemy and has the power to defeat Harry. Voldemort's goal is to take over the magical and non-magical world.Voldemort must be stopped by Harry Potter so evil won't take over the world!
Death Eaters
Death Eaters are a group of evil wizards in the Harry Potter book. The Death Eaters are led by the Lord of Darkness, Lord Voldemort. The Death Eaters aim at total destruction of all non-wizarding families or wizards who are from muggle descent. As well as complete control over the magical and non-magical world.
Dementors
Dementors are souless creatures who guard the wizard prison of Azkaban. They are wraith-like creatures that have no eyes and a large hold in their mouth. They feed on positive emotions, happiness and good memories of the humans forcing them to relieve their worst memories. Dementors are a foul and terrible creature and perfect for Halloween!
Other Costumes
Gryffindor Robes