"Dementors guard the wizard prison, Azkaban."
From the Story
Discovered in Book 3, Chapter 5, The Dementor
"Dementors guard the wizard prison, Azkaban. Tall, hooded creatures, their slimy, scabbed hands look like something dead that has decayed in water. Dementors seem to suck the happiness out of the air, and fill those around them with an intense cold."
Discovered in Book 3, Chapter 20, The Dementors' Kiss
"There is little to see when a Dementor lowers its hood. Where there should be eyes, there is only thin, grey, scabbed skin, stretched blankly over empty sockets. Dementors do have rough approximations of a mouth: they suck at the air around them through a gaping, shapeless hole, making the sound of a death rattle as they do.
At least a hundred Dementors make their way around the lake towards Sirius, Harry and Hermione. The Dementors form a solid wall around them as they get nearer.
The Dementor nearest to Harry stops close to him. It cannot walk through the silvery cloud Harry produces, and makes a gesture as though to sweep the Patronus aside. The Dementor seems to be considering Harry. It raises its hands, and lowers its hood. The Dementor wraps his hands around Harry's neck and forces his head upwards, preparing to give him the Dementors Kiss.
The Dementors are forced away by a bright, silver animal, a Patronus that appears from the other side of the lake."
Discovered in Book 3, Chapter 21, Hermione's Secret
"The Dementors are due to arrive at the castle to perform the Dementors' Kiss on Sirius Black.
The Dementors emerge from the darkness, gliding around the edges of the lake towards Sirius. When the Patronus appears, the Dementors scatter, retreating into the night."