Inspired by Ferretbadger's entry on the role that Snape has to play in the Harry Potter series, I thought I'd put my current thoughts about the mysterious "R.A.B." who stole one of Lord Voldemort's Horcruxes and replaced it with a fake, leaving the note which Harry and Dumbledore discovered on their mission (I'm sure if you're reading this, you remember the note!).
So who is R.A.B. then?
A reminder of the note, found in the fake Horcrux:
"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.", Harry Potter and the Half-Blood Prince, page 569
Well, my theory is encouraged by the fact that Hermione was looking for famous wizards with the initials R.A.B. - however, I think this is the wrong way to go about it. It is addressed to the "Dark Lord", which is what the Death Eaters call him (no-one else calls him that, as far as I remember anyway). So it's from a (former?) Death Eater, or maybe more than one Death Eater, but unlikely given the stuff you had to go through to get to it in the first place (that boat that Dumbledore suggested could only take one person across the lake, for example). And I only know one Death Eater who could have the initials R.A.B. - that's Sirius's brother, Regulus Black, who we know was murdered by or on the instruction of Voldemort... we were told this was because he turned away from him, well maybe destroying the Horcrux and replacing it with a fake was the way he did it. But why would he do it? I guess we have to wait till the last book to find out.
Does anyone other than the Death Eaters call Voldemort the "Dark Lord"? I guess people who agree with Voldemort's ideas but stop short of supporting him fully (for example, like Sirius said his parents did) might do as well, but I'm not sure about this - everyone seems to call him "He Who Must Not Be Named" and other such things.
There is a much better piece of evidence for Regulus that I have to admit I'd missed; it's in "Order of the Phoenix", where there's a "locket that nobody could open" when they're tidying up at 12, Grimmauld Place (I *knew* there was another reason for the tidying up being such an issue). Thanks to the hp-lexicon livejournal account for that discovery!
Another somewhat ridiculous idea
Going with the Death Eater theory though, the only other thing I can think that R.A.B. could stand for was "Rodolphus and Bellatrix" - the Lestranges, but why would they behave like they have done after taking (and we presume, destroying) the Horcrux and replacing it with a fake? And the note says "I" not "We"... so it's one person. Probably.
So my current conclusion is...
I would have concluded, disappointingly, that R.A.B. is someone we don't know about yet. Unless it all relates back to Snape again, like Hermione sort of suggests... unfortunately that conversation finishes rather abruptly.
Although maybe Amelia Bones had a different first name and that's why she was taken out near the start of book 6. But why would she call him the Dark Lord? I have to thank J'au-æmne for coming up with that one, though!
However, this locket clue is a good one! So maybe it is Regulus Black after all, and we'll find out more about him in the final book.
In fact, it seems that people agree with me: this editorial on Mugglenet.com certainly seems to agree.