Regulus Arcturus Black was a decendant from the Black Family. He, like most of his brothers and sisters, was sorted into the house Slytherin at Hogwarts, before becoming a Death Eater. Strangley, later on in the story, turned his back on Voldemort and found a Horcrux. He then attempted to destroy it, being killed by the Inferi in the Horcrux Cave in the process. He was a pure blood, and the younger two sons of Orion Black and Walburga Black.
In Harry Potter and the Half-Blood Prince, at the end of the book harry finds a letter in the fake horcrux, signed by R.A.B, this was actually Regulus Black (as you find out in the last book).