Wanting to use it to revive his family and apologise for his selfishness in the past, Dumbledore lost his common sense, forget that it was a
Gaunt's ring
After retrieving the ring Albus Dumbledore himself fell victim to a curse that, even when contained, would likely have killed him within the year. Once rendered defunct as a Horcrux, the stone was placed inside a Golden Snitch and left to Harry Potter in Dumbledore's will.
Therefore, Dumbledore may have specifically worn the ring—which would indicate his success in tracking down Voldemort's past—both to help encourage Slughorn to return to Hogwarts, and to help persuade him to turn over the correct version of the memory of Slughorn talking to Riddle about Horcruxes.
The curse on the ring refers to a deadly curse placed by Voldemort on Marvolo Gaunt's ring to protect the Horcrux within. The curse was lethal and would have killed Dumbledore almost immediately after he put the ring on if Snape hadn't performed a counter-curse to slow it's spread.
During the last year of his life, Dumbledore's right hand is scarred when he dons Marvolo Gaunt's Ring, which was cursed. Had Snape not intervened with a counter-curse, Dumbledore would have died much more quickly.
He does not find out when he realizes Harry's scar first starts hurting him. Dumbledore and Harry do not really talk about the pain Harry experiences in his scar until the later books, but it's sort of generally accepted that Dumbledore knows about it.
Voldemort did not even realize that Harry and Dumbledore were after his Horcruxes until after 3 of them had been destroyed (Riddle's diary, Slytherin's locket, and Marvolo's ring) and a 4th captured (Hufflepuff's cup).
In order to conjure the avada kedavra curse, you have to want to kill your victim. We all know that Voldemort could easily kill a child without an ounce of remorse... but not Snape. Snape didn't want to kill Dumbledore, and this was why the spell was blue instead of the usual green.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
He wanted to bring his family back (parents and sister) to apologize and tell them how much he loved them. After, all these years, when he saw the resurrection stone in the ring, he forgot the ring was a horcrux and therefore, must carry a dangerous curse on it and put it on.
So when Ron said three, it wasn't a mistake. He meant there were three more horcruxes to destroy before being able to destroy the fourth and final piece which was in fact Voldemort himself.
Dumbledore also gives Harry the Resurrection Stone.
After Dumbledore's death, his will leaves the resurrection stone-containing snitch to Harry. "To Harry James Potter, I leave the Snitch he caught in his first Quidditch match at Hogwarts, as a reminder of the rewards of perseverance and skill," his will reads.
Teddy, Newt's niffler, manages to steal the blood pact vial from Grindelwald at the end of the second film. The vial, contained in an amulet on a necklace string, is the physical manifestation of the blood pact that the two had undertaken in order to prevent them from harming each other.
The memory curse leaves her badly damaged, so Voldemort kills her, sending a piece of his soul into his snake, Nagini, and creating his seventh and final Horcrux. July 1996: Albus Dumbledore destroys Marvolo Gaunt's ring with Godric Gryffindor's sword in his headmaster's office at Hogwarts.
He was sure there were Horcruxes after laying a eye on the diary (or at least started to think Voldemort might have them). What we, however, do know is that by the time book 4 came along, Dumbledore knew that Voldemort had Horcruxes and he also knew Harry would have had to die - expect this had changed.
The Resurrection Stone was one of the three Deathly Hallows, originally in the possession of Cadmus Peverell.
Draco Malfoy's mother Narcissa was cold, cunning and devoted to the Dark Lord. But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.
Her sacrifice gave Harry the power of love, a protection spell sealed with the blood spilled by her death.
He truly believed the Elder Wand had to be passed down through murder, because that is what the legends say. But even if he knew that one only had to be disarmed, Voldemort was still too arrogant to do that. He kills anyone in his way.
Erroneously believing Snape is the master of the Elder Wand and that Snape's death will make him the master of the Wand, Voldemort kills Snape by having his pet snake Nagini bite him through the neck.
Sectumsempra is a curse invented by Professor Severus Snape, during his childhood, when he was known as "The Half-Blood Prince". He created it with the intention of using it against his enemies, and it soon became one of his specialties.
Why didn't Dumbledore think of using a basilisk fang to destroy the horcruxes? He was considered a genius, he knew there was a dead basilisk in the chamber of secrets at Hogwarts and that Harry destroyed Tom Riddle's diary with its fang but instead of doing it the simple way, he kept thinking of other ways.
Not surprisingly, the one person who always seemed to see through the act was Dumbledore. Dumbledore had seen the true Tom Riddle when they first met: the boy who tortured, stole, and completely abused his powers. As a result, Dumbledore never fully trusted him, and Tom never even tried to win Dumbledore's trust.
In an interview, J.K. Rowling revealed that it was the murder of Bertha Jorkins that made Nagini a Horcrux. After Voldemort discovered his Horcruxes were in danger, Nagini was magically protected, described as being encased in a 'starry, translucent sphere'.