Settled in the story
What was the mirror Sirius gave Harry supposed to do?
Sirius's parting gift was a way to call him that Harry would not use, out of love and stubbornness in equal measure. The trail runs from the parcel at Grimmauld Place to a shard in a rucksack and the stranger's eye that watched through it.
This thread gives away a resolved story point
The clue
A wrapped parcel pressed into Harry's hand with instructions he refused to follow, found again at the worst possible moment — and a broken shard of it that kept working for two more books.
The trail
- Book 5 · The goodbye at Grimmauld Place
As Harry leaves for Hogwarts after Christmas, Sirius presses a grubby parcel on him: a mirror, and an instruction — use it if Snape gives you trouble; it is how he will know Harry needs him. Harry packs it away determined never to use it, certain that calling would lure Sirius out of hiding and into danger.
- Book 5 · The last night of term
Too late, in the empty dormitory, Harry digs the parcel out and speaks Sirius's name into the glass. His own face looks back. The pair to this mirror was in Sirius's keeping, and both halves of the conversation are now out of reach. It is the cruellest prop in the series: a telephone picked up after the house has burned down.
- Book 7 · In Memoriam
Packing to leave Privet Drive for good, Harry cuts his finger on a shard of that same mirror, kept all this time at the bottom of his old school bag. He keeps the shard. Grief, filed under equipment.
- Book 7 · The Wandmaker
Through the shard, more than once, Harry has glimpsed a bright blue eye that is not his godfather's. Locked in the Malfoys' cellar, he begs that eye for help — and help arrives, in the shape of a house-elf who had no ordinary way to come. Somebody has the other mirror, and has been watching over him through it.
- Book 7 · The Missing Mirror
Aberforth Dumbledore lays the twin mirror on the bar of the Hog's Head. He bought Sirius's half from Mundungus Fletcher, and he has used it to keep the eye on Harry that his brother no longer could. The eye in the shard was a Dumbledore's — just not the one Harry kept hoping for.
The payoff
The mirrors were a matched pair the Marauders once used to talk through separate detentions. Sirius's half passed through a thief's hands to the one man still running a resistance out of a pub — and the gift Harry refused to use spent two books working anyway, in someone else's keeping.