Regulus Black the First?
Fan speculation has led many people to the conclusion that RAB is Sirius’ brother, Regulus. It seems rather unlikely that an 18-year-old student, who just graduated from Hogwarts (where information on Horcruxes was restricted), would have the knowledge to deal with a Horcrux. We all know what it took to get to the locket. I firmly doubt that Voldemort would have given a young, freshly recruited Death Eater the information either.
However, looking at the Black family tree, I see another likely choice: Sirius’s great-uncle, who was also named Regulus. Regulus Black “the First” was born in 1906 to Sirius Black the Second and Hesper Gamp. His paternal grandfather was Phineas Nigellus, Headmaster of Hogwarts. Looking at the Black family tree, there are plenty of people from which he could have learned about Horcruxes. This includes one Herbert Burke who married his Aunt Belvina. Burke is a familiar name to any Harry Potter fan. Herbert & Belvina had two sons and one daughter. It is likely that one of the sons is the owner of Borgin & Burkes.
Readers of Half-Blood Prince will also remember that a young Tom Riddle worked at Borgin & Burkes. Could Regulus I have visited there at one point and ran into Tom? Furthermore, would this 39-year-old man have been impressed with 19-year-old Riddle?
I believe the answer to both questions is yes. Regulus would have come to his cousins shop to dispose of things of a sensitive nature. Undoubtedly, Tom impressed anyone who met him. To quote Albus Dumbledore, He was one of the most brilliant students Hogwarts has ever seen (Chamber of Secrets, Chapter 18). Harry comments on his beauty several times in Half-Blood Prince. He was charismatic, handsome, brilliant, powerful, etc. How could anybody not be impressed?
Then suddenly he disappears with Slytherin’s locket and Hufflepuff’s cup. An obvious question comes to mind: why would a man with such a promising future disappear like that? Rumors about Voldemort begin to circulate. Remembering Tom’s nickname a person may think he must have just gone away to study.
A decade later, he shows up on Regulus I’s doorstep. He’s surprised, but listens to him as he tells him that he wants to pull the wizard and Muggle further apart. He has a feeling that something is wrong; Tom doesn’t look quite . . . human anymore. But Regulus also sees that Antonin, Mulciber, Nott, and Rosier have joined him so he agrees to help him.
Regulus quickly learns how wrong he was to join. Tom has turned into a monster; he has murdered people and split his soul. Theres no way Regulus can reveal this to anybody; Voldemort would know Regulus betrayed him. He decides the best choice is to destroy one of the Horcruxes.
He realizes, however, that he would know if he destroyed it. He has to hide it so he makes a fake locket and puts it in the liquid. What is he going to do with the locket? He decides to hide it in his nephews house. Orion was always his favorite, and hell hide it for him without asking questions. Grimmauld Palace is so heavily warded that even if Voldemort does find out he will never be able to get in.
This is what I believe happened. Then he arranged his own death in 1959, so he would no longer need to serve Voldemort. However, I also believe that Voldemort killed several of the Blacks, too. Looking at the family tree, we see that several other Blacks died around then, starting in 1959 with Arcturus Black I. Arcturus Black I had three daughters; one of them married a Longbottom, one a Crouch, and one a Weasley. Could Arcturus have been murdered since his daughters all married “light” families?
The next Black to die was Belvina Burke née Black (or Nigellus) in 1962. She was Phineas Nigellus’ daughter, and also the wife of Herbert Burke. Could she have found out something from her son that would make her suspicious of what he was doing? If she was murdered by Death Eaters, it would explain why Burke dislikes Lucius Malfoy, a known Death Eater.
For three years, not a single Black dies. Then in 1965, Lycoris Black dies. Lycoris was Regulus sister. Could she have been trying to find out what happened to her little brother? It is curious to note that their older brother Arcturus the Second (Siriuss parental grandfather) lived to 1991. He not only outlived them, but his son and grandson too.
The next one to die is Charis Crouch née Black, in 1973. Honestly, I don’t think this has anything to do with the Black family. I think it has everything to do with her son, who is most likely Barty Crouch, Senior.
Next are Alphard Black and Dorea Potter née Potter (I don’t think she’s Harry’s Grandmother) in 1976 to 1977. Once again, they were killed because they were “blood-traitors” or married to a Potter. James would have just graduated from school, and was either dating or engaged to a Muggle-born. In addition, he had probably already defied Voldemort once at least, enraging him to the point of wanting to kill anybody named Potter.
The last two deaths that could have been caused by Voldemort – Orion Black and Regulus Black II. Both of them died in 1979. We know that Regulus, the Younger (II), was killed by Death Eaters when he tried to leave, but why would Orion die the same year? I can only guess that Orion tried to tell somebody something that Regulus had told him? Or was he trying to tell somebody about a certain locket his Uncle had given him? Did he die trying to destroy the locket?
Which is more likely: That a scared eighteen-year-old boy would know what a Horcrux was, that Voldemort had them, and devise a plan to hide one, or that his great-uncle would know all that and devise the plan?