Curse on Marvolo Gaunt's ring

"That ring carried a curse of extraordinary power, to contain it is all we can hope for; I have trapped the curse in one hand for the time being —"

- Severus Snape describing the potency of the curse.

A powerful curse was placed upon Marvolo Gaunt's Ring, around the time Lord Voldemort turned it into a Horcrux, as a defence against anyone who might try to destroy it. This curse was designed to quickly kill anyone who dared to wear the ring.

Effects
"The ring, Harry. Marvolo's ring. And a terrible curse there was upon it too. Had it not been — forgive me the lack of seemly modesty — for my own prodigious skill, and for Professor Snape's timely action when I returned to Hogwarts, desperately injured, I might not have lived to tell the tale. However, a withered hand does not seem an unreasonable exchange for a seventh of Voldemort's soul. The ring is no longer a Horcrux."

- Albus Dumbledore explaining the effects of the curse The curse spreads rapidly upon contact with the person and causes them to die in a painful and humiliating way moments after adorning it. However, against wizards with prodigious skill such as Albus Dumbledore and Snape, the effects of this curse may be contained or temporarily halted. In the case of Dumbledore, this led the quarantined section to look blackened and dead. The containment would not hold indefinitely, as the curse would continue to slowly spread and eventually kill him, which, in Dumbledore's case, is estimated to be no more than one year.

Neither the curse nor the damage it inflicts seem to be reversible, as Dumbledore and Snape did all they could, but ultimately could do nothing to save Dumbledore from eventual death. Dumbledore's right hand looked blackened and dead for the last year of his life, seemingly with no improvement. This also hindered his ability to use his right hand, as he had trouble uncorking a bottle containing a memory. All the while his infected hand hurt from time to time.

Placing the Curse
After acquiring his maternal grandfather's prized heirloom ring from his deranged uncle, Riddle wore it openly during his time at Hogwarts. Once learning about the effects and possibility of creating multiple Horcruxes, Riddle no longer desired to wear the ring, and instead turned it into a Horcrux with his father's murder. He then placed the curse on the ring as one of the many precautions before hiding it in its place of origin.

Activation
In the summer of 1996, when Dumbledore found the ring under the rotting floorboards of the Gaunt shack, he saw it immediately as the Hallow that he had desired for so many years, the Resurrection Stone. Wanting to use it to revive his family and apologise for his selfishness in the past, Dumbledore lost his common sense, forgetting it to be a Horcrux which may have been cursed and adorned it on his right hand. Almost immediately, the curse triggered, nearly killing Dumbledore.

However, Dumbledore managed to use his own prodigious skill to contain the curse from spreading any further, which prevented him from dying immediately. He returned to Hogwarts School of Witchcraft and Wizardry quickly and went to see Snape, whose masterful knowledge in the Dark Arts helped him to halt the curse's spread temporarily through a thick golden potion and several incantations.

Unfortunately, Snape claimed that though the curse was contained in his right hand, it would eventually break free of the restraints and spread once again, killing Dumbledore in a painful and humiliating way. Snape also claimed that had Dumbledore managed to return sooner, he might have done more to save him. Upon learning that he had no more than a year to live, and in an attempt at preventing a humiliating and agonising death, he requested Snape to kill him when the time came.

When Dumbledore used Godric Gryffindor's Sword to crack the ring, the curse seemingly disappeared, as he was able to wear the ring to pick up Harry Potter from the Dursleys with no further ill effects. Also, when Harry received the Resurrection Stone that was taken out of the ring, the curse did not spread on contact.

Behind the scenes

 * In LEGO Harry Potter: Years 5-7, this curse is shown to have a further effect: Dumbledore's hand appears to be sentient to some extent. It is shown attacking him at some points, such as before his and Harry's trip to the Horcrux cave, and it becomes detached upon his death and jumps off the Astronomy Tower of its own accord.

Appearances

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1 
 * Harry Potter and the Deathly Hallows: Part 2
 * LEGO Harry Potter: Years 5-7