Symbolism, Imagery, Allegory
Death has always been a large part of Harry Potter's story. After all, the series pretty much begins with a double murder, which results in an orphaned Harry being dropped off at his horrible aunt and uncle's house, like some sort of modern day David Copperfield (funny fact: Daniel Radcliffe starred as a young David Copperfield in a TV movie adaptation of the novel before he made his first Harry Potter movie). But Azkaban kicks the death imagery and symbols up a notch. We have the recurring image of the grim, a black dog/death omen that stalks Harry throughout the novel:
Harry saw something that distracted him completely – the silhouette of an enormous shaggy black dog, clearly imprinted against the sky, motionless in the topmost, empty row of seats. (9.5.35)
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The Dementors are also like Grim Reapers come horribly to life. Death is closely linked to fear via the Dementors in this novel. The lesson that Harry learns is how to combat fear and how to deal with death.
So it's notable that death imagery isn't all negative or terrifying. In fact, this book makes it a point to say that death doesn't have to be scary or awful; it can be a part of life that we accept.
In terms of imagery, how is this point demonstrated? Through the Patronus, mainly. The Patronus, a literal bright white light that scares away the darkness and combats what may be the ultimate evil in this book: fear. A Patronus is kind of like a near-death experience come to life. Patronuses are about hope, and it's extremely fitting that Harry's Patronus was a way to sort of bring his dead father back to life, to combat fear, and to come to terms with death itself.