Harry Potter and the Prisoner of Azkaban2003

Dumbledore:
[to the Hogwarts students] Finally, on a more disquieting note, at the request of the Ministry of Magic, Hogwarts will, until further notice, play host to the dementors of Azkaban until such a time as Sirius Black is captured. The dementors will be stationed at every entrance to the grounds. Now whilst I've been assured that their presence will not disrupt our day-to-day activities, a word of caution. Dementors are vicious creatures. They'll not distinguish.... between the one they hunt and the one who gets in their way. Therefore, I must warn each and every one of you, to give them no reason to harm you. It is not in the nature of a dementor to be forgiving. But you know, happiness can be found... even in the darkest of times...if one only remembers to turn on the light.