Dementors are fictional creatures from the Harry Potter Universe.
 BackgroundDementors are the guards of Azkaban. They are creatures that suck the happiness from people, and leave only that person's worst memories. They can also do a kiss, which is when they suck the actual soul out of a person. There is one highly advanced spell that repels them, and that is Expecto Patronum. The Patronus differs from person to person, but it is usually in the shape of some type of animal.
The dementors are first mentioned in the third book. They are stationed at different parts of England looking for the escaped fugitive, Sirius Black. Harry first meets them on the train, where the train suddenly stops, and the lights go out. It gets quite cold, and Harry starts to hear his mother screaming. Harry passes out and Remus Lupin, the new Defense Against the Dark Arts teacher repels them with a Patronus. After the feast, Dumbledore announces that they are going to be stationed at all of the entrances and secret passages around the school and in Hogsmeade. Harry next meets them during the Quidditch match against Hufflepuff, where hundreds of them come onto the field. Dumbledore gets rid of them, and Harry passes out 50ft above the ground. His Nimbus is destroyed and they lose. After deciding he wants to learn to get rid of them if he meets them, he goes to see if Lupin can help him. Lupin teaches him to cast Expecto Patronum on a bogart dementor. Harry then meets them when Peter Pettigrew escapes from them, and Lupin turns into a werewolf. They don't kiss anyone, and Harry can't handle them. Before he passes out, he sees what he thinks is his father using a Patronus to protect them. they go back into time, and when Harry expects to see his father, he realizes it was him doing it. He does it and saves Sirius, and himself and Hermoine. The dementors next appear in the beginning of the fifth book. Harry and Dudley were walking down an alleyway where two attack them, and Harry uses a Patronus. He gets suspended, but gets back into Hogwarts. After the fifth book, dementors are now joined with Voldemort again, and there is now only innocent people in Azkaban.