Dementor

From Hpwiki

The Dementors are the gaurds of the Wizarding prison Azkaban. The Dementors cause chill and suck the hope and happiness out of people by their mere presence. They can't be seen by Muggles but they can feel their presence. These evil creatures can be repelled with the Expecto Patronum spell. Dementors 'multiply' themselves in cold, dark places. They do not actually breed, but grow like a fungus. Under their black cloaks, they have empty eyesockets and a hole for a mouth. Their skin looks like that of a dead person, but a Dementor was never a living human being and does not possess a soul though it does possess some kind of intellect.

In both Voldemort's first and second rise they have joined the dark wizard, probably to be given souls to prey upon as payment. In the 5th book voldemort is "breeding" an army, which is causing Great Britain to be shrouded in fog year-round.

Their most dangerous aspect is the "Dementor's Kiss", a kiss that sucks the soul out of a person, effectively ending their not merely their life but their very existence and leaving their body living but brain-dead.

In Harry Potter and the Prisoner of Azkaban, a Dementor enters the Hogwarts Express in search of the convicted mass-murderer Sirius Black. It is at this time that Harry realises his weakness towards the Dementors compared to his friends. Luckily for him, Defence Against the Dark Arts teacher Professor Lupin was in the compartment at the time and he managed to ward it off.

The Dementors also play a significant role in Harry Potter and the Order of the Phoenix, where they attack Harry and Dudley at Privet Drive. Harry is forced to use the Patronus Charm to get rid of them, and he is expelled from Hogwarts for doing so. Dumbledore manages to delay this, but Harry must attend a hearing at the Ministry of Magic to debate it.