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 that 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 This curse would rapidly cause the death of anyone who came into contact with it, although in certain cases the spread of the curse could be slowed down by magical means. In the case of Dumbledore, this curse caused injury to his hand and would prove fatal in roughly one year. It was also the type of curse that would strengthen over time.

Placing the Curse
After he had acquired his maternal grandfather's prized heirloom ring from his deranged uncle, Tom Riddle wore it openly during his time at Hogwarts. Once he learned 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 he hid 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 apologize for his selfishness in the past, Dumbledore lost his common sense, forget that it was a Horcrux which may have been cursed, and adorned it on his right hand. Almost immediately, the curse triggered, and nearly killed 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, eventually killing him. 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 agonizing death, he requested Snape to Kill him when the time came.

When Dumbledore used the Sword of Gryffindor 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

 * The effects of the curse on Marvolo Gaunt's Ring is similar, if not identical, to the effects of severe necrosis (premature death of cells) and rhabdomyolysis (death of muscle tissue throughout the body) after a venomous snake bite, usually from the Bothrops asper. The bitten limb also eventually turns black, "withered", and "rotted", unless the victim treated with antivenom shortly afterwards.
 * In LEGO Harry Potter: Years 5-7, this curse was shown to have a further effect: Dumbledore's hand appeared to be sentient to some extent. It was shown attacking him at some points, such as before his and Harry's trip to the Horcrux cave, and it became detached upon his death and jumped off of 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