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. (HBP pg. 609/569)
But who is R.A.B.?
Some fans have guessed that the "B" of R.A.B. could be Borgin or Burke. This is a logical guess from the standpoint that they would want their Slytherin relic back. We don't know Mr. Borgin's first name, but we do know Mr. Burke's first name is Caractacus, so that would lead us to believe that as his initials are C.B., he's probably not R.A.B. However, in history there was an infamous 19th-century murderer and grave-robber who's name was William Burke. Is J.K. dropping a clue here?
Interestingly, besides one other, there are no other characters in the Harry Potter books with the first initial "R" and the last initial "B", except for two people mentioned by Hermione in relation to this question. Right before Dumbledore's "funeral", Hermione tells Harry,
There are a couple of reasonably well-known wizards with those initials -- Rosalind Antigone Bungs ... Rupert 'Axebanger' Brookstanton ... but they don't seem to fit at all. Judging by that note, the person who stole the horcrux knew Voldemort, and I can't find a shred of evidence that Bungs or Axebanger ever had anything to do with him. (HBP pg. 636/593)
Hermione, herself, rules out these people as possible suspects for R.A.B., and as we know, when Hermione says something in a book, she's usually right. (Our H.P. Sleuth rules thanks to our friends at Wizarding World Press state that when Hermione says something, she's usually right, unless she's mad or upset, and whenever Ron says something, he's usually wrong, unless he's joking.)
So, going by Hermione's own logic stated in the book, it has to be someone who knew Voldemort. Do we know anybody like that who also fits the R.A.B. initials? Yes!
Most fans believe it's logical to assume that R.A.B. is Regulus Black, Sirius' younger brother. In Harry Potter and the Order of the Phoenix, while they are examining the Black family tree tapestry, Sirius tells Harry that Regulus was a Death Eater, but was killed by Voledmort:
...he was murdered by Voldemort. Or on Voldemort's orders, more likely, I doubt Regulus was ever important enough to be killed by Voldemort in person. From what I found out after he died, he got in so far, then panicked about what he was being asked to do and tried to back out. Well, you just don't hand in your resignation to Voldemort. It's a lifetime of service or death. (OotP pg. 112/104)
If you put Sirius' description of what happened to Regulus together with the note in the locket, it's not a big leap of logic to assume that Regulus did more than simply want to stop being a death eater. It's likely he knew what Horcruxes were because his parents were very much into the dark-arts, and he found Voldemort's locket horcrux, intending to destroy it. When Voldemort found out, he of course would have killed Regulus, as we know Voldemort did because Sirius told us so.
If R.A.B. is Regulus Black, then what does the "A" stand for? Again, we have some plausible possibilities given to us by clues in the book.
Many fans think Regulus' middle name is Abraxas. In Half-Blood Prince, Draco tells Slughorn that his grandfather's name was Abraxas Malfoy. Since we know, from Sirius' and Harry's trip through the Black family tree in Order of the Phoenix, that the Blacks and the Malfoys are related, it is possible that Regulus was named after Abraxas Malfoy.
But there is an even closer relative who's also a possibility. In the same scene where Sirius tells Harry about Regulus, he also mentions his uncle Alphard, who was good to Sirius, and left Sirius some gold, allowing him to move out of Grimauld Place permanently. If Regulus was Alphard's namesake, it's possible that Alphard had also been a good influence on Regulus, making him see the error of his ways and turn against Voldemort.
So, where is the real locket now? Regulus' note says he intends to destroy it as soon as he can, but we don't know for sure that he did.
If it's anywhere, you'd suspect it's someplace in Grimauld Place, where we assume Regulus lived with his parents. In Harry Potter and the Order of the Phoenix, when they are cleaning out the parlor in Grimauld Place, a locket is mentioned, but only briefly. In the list of nasty things in the glass cabinets, they found:
...a heavy locket that none of them could open... (OotP pg. 116/108)
But is that locket the locket? There's one important clue word that suggests that it is. In its brief mention, J.K. notes that it's a "heavy" locket. In Half-Blood Prince, that word is used to describe Slytherin's locket not once, but twice:
With a howl of rage, Gaunt ran toward his daughter. For a split second, Harry thought he was going to throttle her as his hand flew to her throat; next moment, he was dragging her toward Ogden by a gold chain around her neck. "See this?" he bellowed at Ogden, shaking a heavy gold locket at him, while Merope spluttered and gasped for breath. (HBP pg. 207/196)
She slid back the fine filigree clasp and flipped open the box. There upon the smooth crimson velvet lay a heavy golden locket. (HBP pg. 437/409)
So it's a good chance the locket Harry saw in the cabinet in Grimauld Place was the locket. But, where is it now?
Some fans believe Kreacher has it. Later in Half-Blood Prince, Harry, Ron and Hermione visit Kreacher's nest in Grimauld Place, so that Hermione can leave him a Christmas present. What they saw there included:
In a far corner glinted small objects and coins that Harry guessed Kreacher had saved, magpielike, from Sirius' purge of the house, and he had also managed to retrieve the silver-framed family photographs that Sirius had thrown away over the summer. (HBP pg. 504/445)
So, while it would have been in character for Kreacher to have stolen it, we can't say for sure that he had it last. If he did, it would be bad news for us, because when Kreacher left Grimauld Place using the loophole in Sirius' order to "get out", he went right to Narcissa Black Malfoy or to Belatrix, and the locket would certainly be back in the hands of Voldemort by now.
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