“Why ...why did you put on that ring? It carries a curse, surely you realized that. Why even touch it? ...There is no halting such a spell forever. It will spread eventually, it is the sort of curse that strengthens over time."
-- Snape to Dumbledore (DH33)
Voldemort cast a deadly curse on Marvolo Gaunt’s ring to protect the Horcrux within. The curse would have killed Dumbledore immediately after he put the ring on if Snape hadn’t performed a counter-curse to slow it’s spread. As it was, Dumbledore’s hand was blackened and withered, and he would die within a year (HBP23).
References from the canon
- Tom Riddle used the ring of his grandfather, Marvolo Gaunt, to make a Horcrux to encase part of his torn soul. When Dumbledore later put on the ring, it made his hand wither and blacken, and Snape had to work quickly to keep Dumbledore from dying immediately (HBP23).
- According to Snape, the curse on the ring was powerful and could not be stopped forever, but he managed to give Dumbledore one year to live (DH33).
- The stone in the ring was the Resurrection Stone, one of the Deathly Hallows passed down from Cadmus Peverell (HBP10,TBB/TBB). Dumbledore put on the ring because he thought the stone would give him a chance to see his dead mother and sister again (DH33,DH35). However, the Stone was useless while trapped inside the cursed Horcrux ring because it had to be turned three times in someone's hand in order to work (BLC).