The ring that withers Dumbledore’s hand
The Gaunt ring is both a Horcrux and the Resurrection Stone. Dumbledore finds it, puts it on, and the curse in it kills him on a delay. Harry only learns the second identity at the end.
Setup
1Harry Potter and the Half-Blood Prince · The blackened hand
Dumbledore returns to school with a dead-looking hand and says he was careless with a curse. He will not fully explain the ring yet. He does tell Harry that destroying a Horcrux has a cost.
2Harry Potter and the Half-Blood Prince · The Gaunt shack in a memory
A memory shows the ring on Morfin Gaunt’s hand, a stone set in a gold band with the Peverell mark. Riddle later takes the ring from the shack. Dumbledore treats the object as a Horcrux he has already ruined.
Payoff
Harry Potter and the Deathly Hallows · The Snitch, then the Forest
The stone in the ring is the Resurrection Stone, one of the Deathly Hallows. Dumbledore had hidden it inside the Snitch he left Harry. Putting the ring on was an attempt to see his sister, and the curse is why Snape has to be the one who ends his life on the Astronomy Tower.
