Dementors

"Dementors are among the foulest creatures that walk this earth. They infest the darkest, filthiest places, they glory in decay and despair, they drain peace, hope, and happiness out of the air around them... Get too near a Dementor and every good feeling, every happy memory will be sucked out of you. If it can, the Dementor will feed on you long enough to reduce you to something like itself...soulless and evil. You will be left with nothing but the worst experiences of your life."

- Remus Lupin to Harry Potter (about the Dementors).

The Dementors are dark wraith-like creatures used by the Ministry of Magic to guard the wizard prison Azkaban, which all originally served as the main antagonists of Harry Potter and the Prisoner of Azkaban. They later served as minor antagonists in Order of the Phoenix and supporting antagonists in Deathly Hallows, when they turned against the Ministry and joined Voldemort's army.

Appearance
Dementors appear as tall (around 3 meters), ominous figures who wear black cloaks, and glide above the ground. A Dementor's face has empty eye sockets, covered with scabbed skin. There is a gaping large hole where the mouth should be, which is used for sucking the soul out of the victim. Anyone who has seen what is under their hood is in no fit condition to tell what they saw.

Prisoner of Azkaban
The Dementors are the main later secondary antagonists and the main plot elements of the 3rd film. They are sent by the Ministry of Magic to Hogwarts to guard against Sirius Black. Harry encounters a Dementor in the train and during a Quidditch match, undergoing Patronus training in the process. At the end of the year, Harry and Sirius are encountered by every Dementor in the school. Although powerless at first, a stag Patronus (later revealed to be Harry's future self) saves them and repels the Dementors away. After Sirius Black was proved innocent, the Dementors were removed from Hogwarts.

Order of the Phoenix
The Dementors return as minor antagonists in the 5th film, having sent secretly and illegally by Dolores Umbridge to silence Harry's claims of Voldemort returning. Harry blasts them off with a Patronus charm.

Deathly Hallows
By the 7th film, Dementors were in free roam, with most having sided with Voldemort and the Death Eaters. They appear shortly when they chase the protagonists as they escape from Umbridge in the Ministry of Magic. Later, in the Battle of Hogwarts, an army of Dementors on Voldemort's side invaded Hogwarts, although they are blasted away by a Patronus charm.

19 Years later
When Auror Kingsley Shacklebolt became Minister of Magic, Azkaban stopped using Dementors any longer. It is not stated what happened to them, but it can be assumed that, due to a lack of sustenance, they died out or became extinct or no longer existed.

Powers and Abilities
Dementors can drain the happiness and any other positive emotion 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. The only way to beat a Dementor is to cast a partonus, this patronus can be called by thinking of something happy. Happiness is a Dementor's pure weakness. Though they can't be destroyed, they do spawn like fungi, hinting that they die off on their own. Their own weakness is Patronus Charm.

Harry Potter theme park
The Dementors also make an appearance in the Harry Potter and the Forbidden Journey ride at the Harry Potter theme park in Universal Studios in Orlando, Florida. In this ride, the Dementors are trying to suck out the soul of the visitors until they get chased away by Harry Potter himself.

Trivia

 * WatchMojo.com listed Dementors as among honorable mentions on their Top 10 "Most Evil Harry Potter Characters" list.