Literary Devices

Dementors are a metaphor for depression. It is stated in the earlier that dementors literally suck the life out of their victims. It is also stated that the don't need walls or fences to keep the prisoners in Azkban contained, they are trapped in their own worst thoughts. This can be seen in the physical changes, especially around Hogwarts when the dementors arrive.

Simile:

"But he thought of Ginny, and fear bubbled up like acid in his stomach." (p163)

Harry's fear for what would happen to Ginny, if she was taken by Death Eaters, upsets him to the point that it physically upsets his stomach.

The Sword of Gryffindor is a symbol that represents true courage and bravery, the traits of the house. It is also an allegory, appearing when a true Gryffindor is in need. Harry pulled in from the sorting hat in the Chamber of Secrets and Neville pulls it out in the Deathly Hallows. This proved that the sorting hat had not made a mistake by placing Neville in Gryffindor.