Dementor

The Dementors are soulless creatures[11] considered to be among the foulest beasts on Earth. They are soul-sucking fiends who guard the wizard prison, Azkaban. In the books, Dementors appear to have a generally human shape, approximately ten feet (3.05 meters) in height, but covered in dark, hooded cloaks which reveal only grey, decayed hands. The wraith-like creatures have no eyes, and there is a large hole where the mouth should be. According to the author, they grow like fungi in the darkest, dankest places, creating a dense, chilly fog. This effect is cumulative with the number of Dementors, and large numbers can effectively freeze even a lake with merely their presence. They are unique, compared to other magical creatures, with their ability to glide (fly in the film adaptations) unsupportedly in either the human or wizarding world.

PS. This is the only creature on my blog that is fictional.