Regulus Black's locket

"To the Dark Lord - I know I will be dead long before you read this, but I want you to know that it was I who discovered your secret. I have stolen the real Horcrux and intend to destroy it as soon as I can. I face death in the hope that when you meet your match, you will be mortal once more. - R.A.B."

- Regulus Black's note inside his locket.

This locket was an heirloom of the Black family. With the help of his House-elf Kreacher, Regulus swapped it for Salazar Slytherin's Locket, which was hidden in the seaside cave, in an attempt to destroy Voldemort's Horcrux and bring down the Dark Lord. Regulus was killed in the process when he was dragged underwater by the Inferi, but Kreacher was successfully able to switch the lockets and escape. Regulus' locket was found by Harry Potter and Albus Dumbledore in June of 1997 when they also attempted to obtain the Horcrux. Harry gave it to Kreacher the following August, as an act of kindness.

The defection of Regulus Arcturus Black
Regulus Black had been an admirer of Lord Voldemort from a young age. However, after he finally joined the Death Eaters, he became disillusioned with Voldemort and his goals. When Voldemort cruelly used Kreacher, Regulus' house-elf, to test the defences of one of his Horcruxes, Regulus became determined to undermine the Dark Lord. In 1979, he had Kreacher take him to where the Horcrux was hidden, and ordered Kreacher to switch the Horcrux locket with another locket he had brought with him. Regulus drank the potion in the basin, and was dragged to his death by the Inferi soon afterwards. Kreacher, unable to disobey his master, switched the lockets and escaped, just as he had been ordered to.

However, despite his best efforts, Kreacher was unable to destroy the Horcrux. He kept the locket in the Black family house, and both the real and fake Horcrux lockets lay undisturbed in their new hiding places for over fifteen years.

The hunt for Horcruxes
Albus Dumbledore suspected that Voldemort had created one or more Horcruxes, and spent several years conducting research into this theory. By June of 1997, two Horcruxes had been confirmed and destroyed, and strong evidence as to the creation of four more was discovered. Dumbledore identified the seaside cave from Tom Riddle's childhood as a likely hiding place, and journeyed there with Harry Potter to find the Horcrux within. After making their way past the cave's numerous defences, Dumbledore and Harry obtained the locket and escaped the Inferi.

Later that night, Dumbledore was killed during the Battle of the Astronomy Tower, and Harry found the locket lying next to his body. Recognizing that this locket was not the same as what he had seen in the Pensieve, Harry realized that the "Horcrux" was a fake. Inside the locket was a note signed by "R.A.B." that confirmed that the real Horcrux had already been taken, and that the pain and effort of the ordeal in the cave had been a waste. Harry kept the locket, not because it was valuable, but because of what it had cost to obtain it.

The truth
In August of 1997, Harry Potter, Ron Weasley, and Hermione Granger escaped from Death Eaters at the Wedding of William Weasley and Fleur Delacour, and again on Tottenham Court Road. Seeking a safe place to hide, the trio went to 12 Grimmauld Place. While there, Harry noticed a sign on the door of Regulus Black's bedroom, and realized that he was the R.A.B. from the note. Unable to find the Horcrux in Regulus' room, Hermione realized that they had thrown out the real locket while cleaning out the drawing room almost two years previously.

Harry summoned Kreacher, who explained the story of Regulus' defection and what had become of the Horcrux. Out of pity, Harry gave Regulus' locket back to Kreacher, who was so overwhelmed by the gift that it took half an hour before the trio was able to get him to stop crying. After this, Kreacher became much more respectful towards Harry, no longer resenting having him as his master. He even became more courteous to Hermione and Ron, whom he had previously simply referred to as "the mudblood" and "blood traitor".

When last it was seen, Regulus' locket was dangling around Kreacher's neck as he led the Hogwarts house-elves into the final battle against Voldemort, fighting in the name of his beloved master.

Appearances
Faux médaillon
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Deathly Hallows