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The initials R.A.B. are currently the only clue to the identity of a possibly pivotal character in the Harry Potter books by J.K. Rowling, revealed in Harry Potter and the Half-Blood Prince.
The following note was found in a locket Albus Dumbledore had carried just prior to his death:
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 Chapter 28)
The locket had been recovered by Dumbledore and Harry Potter in the belief that it was one of Lord Voldemort's Horcruxes.
During an interview shortly after publication of Half-Blood Prince, Melissa Anelli from The Leaky Cauldron suggested that R.A.B. could stand for Regulus Black. Rowling replied mysteriously, "Well, I think that would be, um, a fine guess." [1] Many readers have also come to the same speculation independently of Anelli.
Regulus was a Death Eater who tried to disassociate himself from Voldemort. He was killed by Voldemort or on Voldemort's orders. This would indicate that the locket Horcrux was stolen by Regulus and possibly destroyed or stashed away. Chapter 6 of Order Of the Phoenix mentions a heavy, unopenable locket stashed at 12 Grimmauld Place, the Black household. In The Half-Blood Prince, Mundungus Fletcher is seen stealing artifacts of the household so it is possible that the locket is no longer in 12 Grimmauld Place.
Since the note left by R.A.B. addresses Voldemort as "The Dark Lord", and Death Eaters are the only group know to refer to him thus, it is likely that R.A.B. is a former Death Eater. Regulus Black fits the bill.
Evidence Against this includes that even a powerful wizard namely Albus Dumbledore was greatly weakened by trying to take the horcrux, How could Regulus Black take it? The only mention of him joining the Death Eaters, is that he was joining a passing phase, not because he was powerful.
While the majority of Harry Potter fandom belives that R.A.B. is Regulus, there is speculation that R.A.B. is Amy Benson, the girl who Voldemort tortured in the cave where the fake Horcrux was located.
First of all, for this theory to work, you need to assume that Amy Benson is not a Muggle and that she either had a nickname that started with the letter R or her first name was dropped by her, and that first name started with R. The evidence is that by the nature of the note, you can tell that R.A.B. knew Voldemort personally. Amy Benson would fit that, as she was tortured by Voldemort as a child. The next piece of evidence is that Amy Benson would definitely know the location of the cave, while there is no indication that Regulus Black would.
It is also possible (but unlikely) that R.A.B. is Ludo Bagman's father, mentioned briefly in Goblet of Fire.
All we are told of Bagman Sr. is that he was an old friend of Augustus Rookwood, a Department of Mysteries employee and Death Eater. Ludo Bagman is known to have collected information for Rookwood during the First War and later claimed he did not know Rookwood was a Death Eater. Ludo was acquitted largely due to his popularity as a Quidditch player. His father could have been a Death Eater who subsequently betrayed Lord Voldemort and stole the locket Horcrux.
If this theory is true, then Harry will have to find Ludo Bagman, who is on the run due to massive gambling debts.