Now I know anybody who has read the sixth Harry Potter book all the way through has seen the ending where Harry finds the message about the Horcrux necklace from somebody named R.A.B. My best friend and I were all like "What the crap? Who the hell is RAB?"

But I finally figured it out. Anyone else aside from Dumbledore who knew about the Horcruxes Voldemort was making had to be a Death Eater who overheard something they shouldn't have. So today I was thinking, which Death Eater has the initials RAB? Then, I remember Sirius's little brother. The person who stole the necklace Horcrux with the initials R and B is Regulus Black!

If you see a flaw in my theory let me know.