<p>“In the (unlikely?) situation that RAB isn’t Black or someone else with those initials, what if R A and B are three people? I still find it incredibly hard to believe that someone who was never very impressive (in Sirius’s, Voldemort’s and Slughorn’s opinion) was able to do what not even Dumbledore could do alone. That Horcrux’s defense was designed for more than one person, no?”</p>
<p>I think it is only one person. Read the note again… It sounds like it is from one person.</p>
<p>“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.”</p>
<p>He keeps saying I so it has to be from one person. It could be Black because Sirius said that his brother probably got in too deep. This seems like a likely reason why he would die and Rowling had to have added him in for some reason.</p>