Defence Against the Dark Arts is a subject taught at Hogwarts School of Witchcraft and Wizardry. Its purpose is to teach students how to defend themselves against unfriendly witches and wizards, as well as dark creatures. Of the eight known Professors in the subject, half were Death Eaters and another almost ended up using an Unforgivable Curse on Harry Potter.
 The Jinx
Tom Riddle applied twice for the job of Defence Against the Dark Arts professor. Once to Armando Dippet when he was eighteen and once a few years later to Albus Dumbledore. He was refused the post both times, on the first instance for being too young, on the second because Dumbledore knew what sort of a wizard Riddle was becoming. Although Voldemort never specified what subject he wanted to teach the second time, it was apparent that it was Defence as ever since that time no Defence Against the Dark Arts teacher ever lasted more than a year. J. K. Rowling has since said that after Voldemort's death the jinx was lifted.