In the harry potter story - Why did Dumbledores hand turn black?

Answer 1

There is a ring that Voldemort made into a horcrux. Dumbledore put on the horcrux and there was a dark charm on it that was made to poison the body. Dumbledore would have died right away if Snape hadn't done a charm to slow the process. So Snape and Dumbledore made a deal for Snape to kill Dumbledore when the time comes so he would gain the full trust of Voldemort. The elder wand became loyal to Malfoy because it changes ownership with the person who DISARMS its previous owner.

Since Harry later disarmed Malfoy at Malfoy Manner, the wand became loyal to him and this is why the Elder Wand failed Voldemort in the final movie. However, the reason Voldemort is unable to kill Harry is not because the Elder Wand failed him. It was because when Voldemort was resurrected, he used Harry's blood, so that part of him runs through the Dark Lord's veins and Harry's mother's protection charm disables him from harming Harry. Therefore, when Voldemort strikes Harry in the final movie with a death curse, he instead destroys the part of his soul that was in Harry. The night he tried to kill him as a baby, he accidentally made Harry into a human Horcrux.

Answer 2

because the curse that the ring, one of the horcruxes, contained. Snape managed to stall the progress of the curse but it would only had lasted for a year, maybe two if they were lucky.