Dementors are a fictional magical species seen in Harry Potter and the Methods of Rationality.
Dementors are a magically created incarnation of Death. They bring decay wherever they go (corroding through the toughest metals in a few hours) and make any sentient being who approaches them fall into a deep state of depression called Dementation. They are also able to permanently make a sentient being into a mindless vegetable, the closest one can get to death without actually dying; this is called the Dementor's Kiss.
Although they are invisible, they project the illusory appearance of a rotting human corpse to their observers — the more an obersever fears the Dementor, the more decayed and ghastly the corpse will look. If one is aware of Dementors' true nature as incarnation of Death, they will see past the illusion to only see a void. At any rate, most Dementors are clothed in dark robes given to them by the Ministry of Magic to avoid forcing people to look at their corpselike illusions all the time. Dementors have no mind of their own, and act based on the expectations of sentient beings around them. Because of this, many will swear a Dementor talked to them or communicated in some way. Knowing a Dementor's true nature similarly makes one deaf to their words.
They can be repelled by a Patronus Charm, which embodies the concept of ignoring death by thinking about the happy parts of life. As such a method, in truth, does not remove the threat of eventual death, Patroni cannot destroy Dementors completely. Because of their inability to destroy them, wizards concentrated Dementors on Azkaban Island, using them as wardens for their prison. In 1991, however, Harry Potter rediscovered a more powerful version of the Patronus Charm, cast with the thought that death could one day be defeated rather than with unrelated happy thoughts. This Patronus 2.0 is capable of permanently burning down Dementors.