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A Horcrux — destroyed by Dumbledore, but the curse on it is killing him.
Dumbledore destroyed the ring Horcrux over the summer but was cursed in the process. The curse has blackened and withered his right hand. Snape has contained the curse to the hand but cannot stop it — Dumbledore has perhaps a year to live. The stone set in the ring is the Resurrection Stone — the second Deathly Hallow — but Dumbledore keeps this to himself.
A gold ring set with a large cracked black stone — the Resurrection Stone, though Dumbledore doesn't reveal this yet.
Also known as: the ring, Gaunt's ring, Marvolo's ring