Dementor

Dementors are considered the most foul, repulsive creatures in existence. Their appearance has been described as skeletal, covered by a dark cloak that envelopes their entire frame. Very few have seen a dementor's actual face; those that do are, in general, incapable of talking about it afterwards.
The diet of a dementor is rumored to be human souls. While this has not been confirmed by the Powers-That-Be, it is known that after being exposed to a dementor, a wizard will lose all feelings of happiness or contentment. As such, the largest known dwelling place on Azkaban Island is considered one of the worst places in the civilized world to visit.