Why Was Dumbledore Dying

Why was Dumbledore’s hand turning black?

After Dumbledore returned to Hogwarts, he used Godric Gryffindor’s Sword to crack the Resurrection Stone so as to destroy it as a Horcrux. While the act did destroy the ring as one of Riddle’s Horcruxes, it did not stop the fatal curse. The curse gave Dumbledore’s hand a withered black look, as though it had died.

How did Dumbledore get poisoned?

Hoping to revive his dead family members (particularly his sister Ariana Dumbledore), he disregarded the ring’s status as a Horcrux and put it on. This enacted the ring’s deadly curse, and it began to quickly spread through Dumbledore’s body, starting with the hand on which he had put the ring.