The Stag on the Lake — The Trinity
Toward the end of Harry Potter and the Prisoner of Azkaban, Harry finds a way to produce a corporeal Patronus charm strong enough to ward off more than 100 Dementors. However, Harry is able to do this only after he has come to believe, if only for a very short amount of time, that his deceased father had returned to perform it. The Patronus charm is immensely difficult and can be produced only using the force of the strongest memories.
In this scene by the lake, Harry is in two places at once thanks to Hermione’s Time-Turner. During his first encounter with the Dementors, Harry is rescued by the producer of a powerful, stag-shaped Patronus. Harry catches a glimpse of this figure and comes to believe it was his father. This lost and orphaned boy, who sat for hours in front of the Mirror of Erised wishing for his deceased parents, finally has an hour in which he believes his father has returned from the dead to reclaim, protect, and care for him.
The words used to produce this particular charm are “Expecto Patronum,” which can be translated as “in expectation of the father.” It is with these words and on the grounds of the fresh memory of having believed in his returned father that Harry is eventually able to produce a fully corporeal stag that drives away the Dementors. It is this second Harry that rescues the earlier one.
The Patronus stag canters back across the water and is said to stare at Harry. Harry stares back and names it Prongs. It is not hard to notice that Harry’s Patronus, the image of his authentic identity, just happens to have his father’s animigus shape and nickname.
So who is it that is looking at Harry when the silver stag stares at him? Rowling leaves this in delicate and poetic indefiniteness. The stag is, of course, both Harry and James, mysterious and paradoxical as that may be. This paradoxical unity of father and son walks across water and yet emerges out of Harry himself. An external savior does not rescue Harry. The rescue, if it can be called rescue, comes from within the very spirit of the boy himself.