Dementors

The Dementors used to work for the Ministry of Magic, to guard the wizard prison, Azkaban. Later in the Harry Potter series, they turn against the Ministry and join Voldemort's army.

Appearance
Dementors appear as tall, ominous figures who wear black cloaks, and glide above the ground. Anyone who has seen what is under their hood is in no fit condition to tell what they say.

Powers
Dementors can drain the happiness out of any area. In doing so, they can trap their victims within their own miserable thoughts. Their presence makes any area cold. When they get close enough to their victim, they perform something called a Dementor's Kiss, where they suck their victim's soul out.