Regulus Arcturus Black was the younger brother of Sirius Black. He attended Hogwarts School of Witchcraft and Wizardry where he was proud to have been Sorted into Slytherin House, playing Seeker for the Slytherin Quidditch Team. He joined the Death Eaters aged 16, but had a change of heart. He died attempting to find one of Lord Voldemort's Horcruxes, leaving his faithful house-elf Kreacher to destroy it. In its place he left a different locket containing a note to Voldemort signed 'R.A.B.'.
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.
- Regulus Black's note to Lord Voldemort