Curse on Marvolo Gaunt's ring

When Lord Voldemort took his grandfather's heirloom ring from his uncle, he eventually placed a powerful curse on it when he turned it into a horcrux, as a defence against anyone who would try to destroy it. This curse was designed to infect anyone who dared to wear the ring (or even touch it) to die quickly.

Effects
The curse spreads rapidly upon contact with the person, and causes them to die in a painful and humiliating in moments after adorning it. However, with prodigious skills such as Albus Dumbledore and Severus Snape, it may be contained and temporarily halted. This would lead the quarantined section look blackened and dead. The containment would eventually leak, causing it to spread and continue to kill the victim.

History
In 1996, when Albus Dumbledore found the ring under the rotting floorboards of the Gaunt Shack, he saw at once as the Hallow that he desired for 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 very well to be cursed, and adorned it on his right hand. Almost immediately, the curse triggered, and almost killed Dumbledore.

However, Dumbledore managed to use his own prodigious talents to contain the curse from spreading any further, which prevented him from dying within moments. He returned to Hogwarts quickly, and went to see Severus Snape, whose masterful knowledge in the Dark Arts managed to halt the curse's spread temporarily through a 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 were Dumbledore managed to return sooner, he might have done more to save him. In turn Dumnbledore knew that he has no more than a year to live, and to prevent himself from dying in such an unpleasant death, he requested Snape to kill him when the time comes.

Appearance

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)