Jinx on the post of Defence Against the Dark Arts teacher

"You see, we have not been able to keep a Defence Against the Dark Arts professor for more than a year since I refused the post to Lord Voldemort."

- Albus Dumbledore

This jinx was cast by Tom Marvolo Riddle, who was by the time known exclusively as Lord Voldemort, to curse the Hogwarts school's Defence Against the Dark Arts class to ensure that none of its applicants could hold their post for more than one year. It is unknown what the requirements and incantation of this jinx are, or if it could be used for other school subjects or other establishments and careers.

History
Upon his graduation from Hogwarts, Tom Riddle asked the then Headmaster, Armando Dippet, for the post as Professor of the Defence Against the Dark Arts class, due to his love for Hogwarts and passion for the Dark Arts. Professor Dippet rejected Riddle's application, because he was too young at eighteen years old, but offered him to reapply after a few years.

Over a decade later, when Riddle resurfaced as Lord Voldemort, he returned to Hogwarts to ask for the job once again. By this time, Albus Dumbledore had taken over as Headmaster, and knowing of Voldemort's ill-intentions, rejected his application. Although Voldemort was merely using the application as an excuse to return to Hogwarts to do his own dark researches, recruit more followers, and to hide his diadem Horcrux, he was still infuriated to be denied his chance to return to the school. In revenge, Voldemort employed this jinx to ensure that no applicant of the Defence Against the Dark Arts post could keep their position for more than one year.

A secondary and more strategic reason for jinxing the post could be Voldemort's wish to reduce the quality of the subject, as its purpose was directly opposed to his personal goals. To disrupt the the teaching by an annual change of staff would certainly have a long time effect in his plans, as future students would be less capable of self-defence.

Known victims of the jinx
Once Voldemort placed his jinx on the post, around the end of each school year, the teachers of the class are forced to leave due to one reason or another, from the effects of the jinx.
 * Quirinus Quirrell: 1991–1992 school year, died when he suffered excruciating pain from Harry Potter's love-protection, as well as when Voldemort's fragmented soul left his body.
 * Gilderoy Lockhart: 1992–1993 school year, lost his memory due to a backfiring Memory Charm with a broken wand, sent to St Mungo's Hospital for recuperation, never fully recovered.
 * Remus Lupin: 1993–1994 school year, revealed by rival teacher Severus Snape to be a werewolf, forced to resign due to parents' protests and safety of the school.
 * Barty Crouch Jr.: 1994–1995 school year, Alastor Moody claimed to be only teaching a year before retiring; turned out to be Barty Crouch Jr. in disguise, Barty received the Dementor's Kiss, Moody did not teach at all.
 * Dolores Umbridge: 1995–1996 school year, removed when Cornelius Fudge was forced to accept that Lord Voldemort had indeed returned, leading to the abolishing of all of Umbridge's Educational Decrees.
 * Severus Snape: 1996–1997 school year, escaped when he murdered Albus Dumbledore, which was secretly planned with the Headmaster to spare him the pain of a curse, abandoning the post in the process.
 * Amycus Carrow: 1997–1998 school year, arrested after Voldemort's demise for membership in the Death Eaters organisation.

The jinx lifted
After Voldemort's downfall, the jinx was broken and Hogwarts was able get a permanent Defence Against the Dark Arts teacher.

Behind the scenes

 * In the films, several of these teachers suffer a slightly different fate.
 * Quirinus Quirrell, instead of dying of excruciating pain, was turned to stone upon being in contact with Harry, and crumbled to death.
 * Barty Crouch Jr. was not noted to receive the Dementor's Kiss, but he certainly fell into the custody of the authorities.
 * Dolores Umbridge was suspended for suspicions on her activities in school, with an impending inspection.

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)