To the Dark Lord
I now 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. (HBP28)
When attempting to destroy Voldemort’s Horcrux made from Salazar Slytherin’s locket, Regulus Black (R.A.B.) left behind a fake one in the sea cave (HBP26).
- Young Tom Riddle knew that Salazar Slytherin's locket had been passed down in the Gaunt family for generations. He discovered that mother sold it to Mr. Borgin in Diagon Alley for almost nothing before she died. Riddle tracked it down to the new owner, Hepzibah Smith, and killed her for it (HBP20). That was the "real" locket that was made into a Horcrux.
- Regulus Black became a Death Eater during the first war with Lord Voldemort, and it made his pureblood family proud, according to his brother Sirius. He regretted joining soon after, but was trapped by his decision (OP6).
- A year later, Voldemort told Regulus to order his house-elf, Kreacher to come with him to a sea cave, where the Dark Lord made Kreacher drink all the potion in a stone basin. Voldemort put Slytherin's locket into the basin and refilled it, before getting back in the boat and leaving the house-elf to die. But Kreacher had been ordered to return home, so somehow he survived the poisoning and walked through water full of Inferi before arriving back at Grimmauld Place (DH10).
- Regulus was disturbed by Voldemort's callous treatment of the house-elf, and soon after he figured out that the Dark Lord had created a Horcrux to help him live forever. He bravely made a choice to go back to the cave, taking Kreacher with him along with another gold locket from the House of Black. This time, Regulus drank the potion while Kreacher removed the Horcrux locket, replacing it with the fake one. Regulus died in the cave after falling into the water, and Kreacher went home and tried to destroy the real locket, but the curse on it was too strong (DH10).
- Dumbledore figured out where to search for the Horcrux when he remembered that Tom Riddle had terrorized two children from the orphanage there. He and Harry traveled to the sea cave where Dumbledore drank the potion, and Harry removed the fake locket (HBP26).
- After Dumbledore was killed during the Battle of the Tower, Harry went to see his body lay and found the locket on the ground. Immediately he could tell it was not Slytherin's real locket, which he had seen in a Pensieve memory. Inside was the note from R.A.B. to Voldemort, stating that he intended to destroy the real Horcrux (HBP28).
- Harry figured out who R.A.B. was when he saw the sign on the bedroom at Grimmauld Place: Do Not Enter Without the Express Permission of
Regulus Arcturus Black (DH10).
- Kreacher told Harry the story of his master Regulus, and Harry allowed him to keep the fake R.A.B. locket, which made the house-elf so emotional it took half-an-hour to calm him down (HBP10).