Night of the Dementor attack

The Night of the Dementor attack occurs on the journey to Hogwarts at the start of Harry Potter's third year. As the Hogwarts Express speeds north, the weather worsens into a thick, hammering rain, and the train unexpectedly slows and stops. All the lamps go out, plunging the passengers into total darkness. The compartment door slides open, and a cloaked figure towers in the doorway, its face hidden beneath its hood. A hand protrudes from the cloak, glistening, grayish, slimy-looking, and scabbed, like something dead that had decayed in water, before being withdrawn. The creature draws a long, slow, rattling breath, as though trying to suck something more than air from its surroundings.

The Dementor's Effect on Harry

An intense cold sweeps over the compartment, penetrating deeper than skin, reaching inside Harry's chest and heart. Harry's eyes roll up into his head; he cannot see and feels as though he is drowning in cold, with a rushing in his ears like water. He is dragged downward, and from far away he hears screaming — terrible, terrified, pleading screams. He wants to help whoever it is but cannot move; a thick white fog swirls around and inside him. He collapses from his seat onto the floor, rigid and twitching, while Ron and Hermione, though frightened, do not lose consciousness. Professor Remus Lupin, who has been sleeping in the compartment, wakes and steps over Harry, walking toward the Dementor with his wand drawn. He says, "None of us is hiding Sirius Black under our cloaks. Go," but the Dementor does not move. Lupin then mutters something, and a silvery thing shoots from his wand at the creature, which turns and glides away. The lights come back on, and the train resumes its journey.

Aftermath and Significance

When Harry comes to, he is sick and covered in cold sweat. Lupin gives him a large piece of chocolate, explaining that the creature was a Dementor, one of the Dementors of Azkaban. The event immediately establishes the Dementors as a terrifying presence at Hogwarts, as Professor Dumbledore later announces that they are stationed at every entrance to the grounds on Ministry of Magic business, warning that nobody is to leave school without permission. The attack also reveals Harry's particular susceptibility: he hears the last moments of his mother's life, her attempts to protect him from Lord Voldemort, and Voldemort's laughter before he murdered her. This trauma, as Lupin later explains, is why the Dementors affect Harry worse than others — because there are horrors in his past that the others do not have. The event sets the stage for Harry's subsequent lessons in the Patronus Charm, his struggle to produce a corporeal Patronus, and his eventual success in conjuring a stag Patronus that drives away the Dementors during the Quidditch match against Ravenclaw and later during the rescue of Sirius Black. The Dementors' presence also leads to the tightening of security around the school, the search for Sirius Black, and the eventual removal of the Dementors after they attempt to administer the Dementor's Kiss on an innocent boy.