My name is Regulus Black. I have been one of the Dark Lord's supporters since the beginning.
We were a family of purebloods, the Blacks, and we dutifully married other pureblood families such as the Malfoys and the Lestranges. I never questioned my duty to the family, even where the Dark Arts were concerned.
I mocked my brother for becoming a Gryffindor. Our whole family had been Slytherins, since time immemorial, and he disgraced us by becoming a Gryffindor. He and that ridiculous James Potter and that werewolf, Lupin. And ridiculous Peter Pettigrew. Well in the end, we found out where Pettigrew really belonged. I hated them all. Lily Evans too. The "Marauders" got into trouble, but they were Gryffindors through and through. Me, I wouldn't have belonged anywhere but Slytherin.
So, he burned the mark onto my arm, and I came when I was called. He trusted me, more than the other Death Eaters knew. He confided in me about the Horcruxes. Oh yes. There had to be two to get Slytherin's locket, and he trusted me alone as his companion. I went with him to put it there, saw the Inferi in the lake. He trusted me, oh yes, he trusted me.
Then I heard about the plot to kill the Potters. I don't know why, but it infuriated me that he would kill a small child. We'd killed whole families before. Perhaps because they were my brother's friends, and I still had some vestige of loyalty to him from childhood. I don't really know why. But, I went that very night to the cave, and switched the lockets. I left a note, so he'd know who'd done it, and I signed it R.A.B.
I hid it, in a locked drawer at Grimmauld Place, the Black family mansion. I didn't have time to destroy it. He knew I'd taken it, and he was hunting for me. I wanted to destroy the others as well, but I'm running out of time. I'm in hiding now, and don't dare leave or he will find me, and I'll be unable to complete my mission.
I've sent a message to Dumbledore, and he now knows the identity and location of most of the Horcruxes. I never could discover what Voldemort would use as the sixth. Perhaps, wisely, he didn't trust me that much. Even if I betrayed him, he would still have part of his soul, and he could survive, somehow.
He's coming.