Dementors

The Dementors are the guards of the Wizarding World prison Azkaban. They are some of the foulest and darkest creatures to inhabit the world of the living. They feed off happiness and spread sadness and despair wherever they go.

The Dementors were introduced in the book Return of Sauron; Lone Traveller as a group of them intercepted the Hogwarts Express and attacked Freedom, who was travelling to Hogwarts with the train. She was saved by her friend Remus Lupin, who told her what the Dementors were and that they had disappeared from Azkaban a couple of days earlier. By the looks of it they had travelled into the Magic Realm and were guarding the border between Hogwarts and the Northern Wilderness, to ensure that none of the witches and wizards at the castle could travel south to aid their friends in Sauron's war against the humans and Elves.

The Dementors disappeared after Sauron was, once again, destroyed, only to show up at Andernilwen's court in Valinor. She never seemed to use them against her prisoners, though, as she wanted to break them herself, but she did send them out to find Sandra in the Republic. Only a small group managed to travel through the universe though and once there, Sandra defeated them by remembering the Patronus-spell, which is the only spell that works on Dementors. By using the Golden Sword in the casting of the spell, she ultimately killed the remaining Dementors, thus resulting in the destruction of the race.