A Dementor is a harmful creature-like magical phenomenon.
The only “real”, material feature of a Dementor is a black, tatterred cloak of unknown origin in which they are shrouded. The way the cloak is shaped when worn by a Dementor suggest a material, vaguely humanoid shape, but that is as far as one can go. Most people see a corpse (on a variable level of decay) under the cloak, but that is an illusion imposed on the viewer by the Dementor's magic, and not its true form. A viewer informed of a Dementor's true nature, or at least aware that the Dementor is trying to impose a fake image on his mind, will instead see nothing at all under the cloak. Muggles cannot see the Dementor at all, whether or not they know about its true nature. (It is unknown if they can still see the cloak).
Dementors seem to possess Mimetic Intelligence.
Dementors are believed by Harry James Potter Evans-Verres to be a “shadow” that Death casts upon the living world, through some weird, yet unknown law of magic.
Quirrelmort once told Harry about "a ritual to summon Death", also telling him that "the ritual to dismiss Death had been lost". Once Harry rediscovered the "ritual to dismiss Death", which he believes to be the True Patronus, he came to the conclusion that the ritual Quirrelmort had told him about had the effect of creating Dementors. This is very likely, but has not been actually confirmed.
The ritual demands a rope which has hanged a man and a sword which has slain a woman.
The way Dementors are repelled is based on their "true nature" as Death's physical embodiment. A Patronus can repell the Dementor, because it is fueled by the fact of thinking about the good side of life instead of thinking about Death. A True Patronus can destroy them, because it is fueled by the thought that Death does exist and that we must face it, but that we will one day defeat it.
Notes and references
- The cloak is not part of the creature: it is left behind when a Dementor is destroyed through True Patronus exposure.
- In canon, Dementors formed spontaneously. In the unofficial spin-off of HPMOR Following the Phœnix, Harry reluctantly performs the ritual in order to use the Dementor to kill Tom Riddle.