Dementor

Dementors (Idolon Dolorus) are a species of idolonid sapient humanoids.

Physical Description
Dementors are tall and gaunt humanoids. They have skull-like heads with wide toothless mouths and empty eye sockets. Their skin is greyish and scabby. Dementors traditionally shroud themselves in tattered black robes with a hood pulled back over their face, due to their fearing sunlight.

Society
Unlike Banshees, Dementors have a chaotic and primitive society where the toughest one wins. Many scholars believe that Dementor's loveless, individualist lifestyle is due to the fact that their asexual reproduction was inconductive to bonds being formed among individual Dementors in their early life.

Reproduction
Dementors breed by surrounding themselves with large amounts of a special fog which they exude. New individuals, already fully formed, will gradually congregate from the mist. Dementors usually breed in large groups.

Lifespan
Dementors are functionally immortal, as they cannot die of old age and are immune to nearly all diseases. If a Dementor's body part is severed, it will vanish into smoke and then slowly grow back over the course of a few weeks. This process can be repeated indefinitely. In the case of the head being cut off, it is the body who will vanish, and a new one will regenerate from the stumped neck.

Magic Absorption
Dementors have the ability of sucking out magic in a small radius. Though Dementors seem unable to cast spells as such, they use this energy to sustain themselves, and never eat conventional food. Their ability to sense magic is also believed to be what replaces their absent eyes.

Gliding and Flight
Dementors, in spite of possessing legs, usually move by gliding, which is no doubt powered by their aforementioned magic. The strongest Dementors are even capable of rising far above the ground and actually flying, though this is very tiresome and they only do so when a kill is imminent.

Emotion Absorption
Though the specifics are hard to study, Dementors project an aura of unhappiness, appearing to quite literally suck out happiness and other positive emotions from their victims. A general aura of gloom is sure to surround a Dementor, but they can willingly focus this power to absorb happy memories entirely.

Dementor's Kiss
The most terrifying power of Dementors is the Dementor's Kiss. When performing a Dementor's Kiss, a Dementor will seal their lips onto another being's and suck out their soul, which they will absorb and destroy to feed on the happiness and magic it contains. Dementors are the only thing in the universe known to completely destroy a soul, rather than simply sending it into the Afterlife, or, at worst, splintering it.

Ancient Times
Dementors originally dwelt in small tribes in the Northern regions, to which their biology is best suited. These early Dementors cohabitated more or less peacefully with tribes of other sapient beings on whom they occasionally preyed, without usually causing long-term damage (as it was essential to their survival that the few beings would be able to reform more magic and happiness to sustain them later on).

Middle-Ages
In the Middle-Ages, wizards developped a defence against Dementors; namely, the Patronus Charm, which focuses pure happiness into an unbreakable physical force whose presence seems to cause pain to Dementors. Kept away from their prey of choice, most Dementors eventually withered away and starved. Only a few remained in areas where wizards were scarce and muggles were isolated enough that news of the Dementor's existence did not reach wizards.

Azkaban Island
In the early 1500s, a phenomenally depraved Dark Wizard by the name of Ekrizdis built himself a magical fortress on the island of Azkaban and began to lure muggle sailors there to practice his dark magicks on them for his sadistic enjoyment, before eventually killing them. Among the torments he imposed upon the poor souls was constant exposure to a colony of Dementors whom he had bred from a handful speciments retrieved from the Russian wilderness.

Guardians of Azkaban
When Ekrizdis died, the concealment charms placed upon his fortress of horrors failed, allowing the mysterious location to be investigated by the British authorities. Though they managed to destroy most of Ekrizdis's devilish creations, they were unable to dislodge the Dementors, who put up a fierce resistance. Deciding to kill two birds with one stone, Wizarding Britain began to ship their hardest-to-keep prisoners to Azkaban, as the Dementor's magic absorption and despair aura would greatly hinder any escape attempt. The Dementors remained the guards of the wizarding prison for centuries. Azkaban however was a living hell. Most prisoners went mad, trapped within continues recollections of their worst memories and a vast graveyard had been erected for those who had perished within the walls, not wishing to continue to live in this torturous hell.

Voldemort's Wars
When the British Dark Lord Voldemort began ascending in the second half of the 20th century, he managed to sway the Dementors to his side in spite of his inability to produce a Patronus Charm, promising them the souls of thousands of Muggles if they were to help him conquer the world. After Voldemort's apparent death, the Dementors returned to Azkaban, seemingly reformed, tormenting those very same Dark Wizards they had been aiding — only to switch sides again when Voldemort came back from the dead in the 1990's. After the Dark Lord's final defeat, the murderous guards were finally removed from Azkaban, most of them taken to the Ministry under threat of Patronuses and subsequently thrown through the Veil of Death.

Ministry Classification
Dementors are classified by the British Ministry of Magic as Non-Beings. Although the normal threat-assertion nomenclature is only meant to reach out to five Xs (for "known wizard killer"), Dementors' threat level is XXXXXX, as their soul-destroying ability are even worse than certain death.

Canon

 * Dementor on the Harry Potter Wiki