"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."
-- Regulus Black in his note to Voldemort (HBP28)
Regulus was the younger brother of Sirius Black. Unlike Sirius, Regulus was favored by their parents because he shared their overweening pride in their heritage and their belief in pure-blood supremacy; indeed, Regulus became one of Voldemort’s Death Eaters around 1977 at the age of sixteen.
Sirius thought that Regulus was killed by Death Eaters because he panicked and tried to leave, calling him “Stupid” and implying that he was a coward. Sirius told Harry dismissively “I doubt Regulus was ever important enough to be killed by Voldemort in person” (OP6). Remus Lupin believed the same about Regulus, that when he fled the Death Eaters he “only managed (to live) a few days as far as I can remember” (HBP6).
In reality, Regulus began to have a change of heart when he saw first hand what the Dark Lord was doing.
In 1979, Voldemort demanded the use of a house-elf, and Regulus sent Kreacher, a family elf that he was very fond of. However, the elf was left behind to die by Voldemort on the secret island in the sea cave where he’d hidden the locket Horcrux. Regulus had commanded Kreacher to come back, however, so the elf was compelled to return using house-elf magic. The story Kreacher told Regulus was the last straw and the young man became determined to destroy Voldemort (DH10).
From arrogant hints the Dark Lord had dropped, Regulus figured out what the locket was – an evil Horcrux. Regulus ordered Kreacher to return to the island with him, substituted a fake horcrux for the real one, then told Kreacher to take the real Horcrux locket and destroy it. Regulus, who had drunk the potion in the stone basin to get to the locket, was attacked and drowned by the Inferi in the underground lake (DH10). He had forbidden Kreacher to tell anyone in the family what had happened, so they would be protected by not knowing.
Harry traveled to the sea cave with Dumbledore and retrieved the gold locket which Regulus and Kreacher left in the stone basin. But it was only after Dumbledore’s death that Harry realized it was not Slytherin’s locket at all, but a replacement (HBP28). Inside the locket he found the defiant note from the mysterious “R.A.B.” who had apparently figured out that Voldemort had a Horcrux. Later while hiding with Hermione and Ron at Grimmauld Place, Harry saw the name “Regulus Arcturus Black” and figured out he was R.A.B., then he asked Kreacher to tell him what he knew about the fake locket and the Slytherin’s real Horcrux locket. (DH10).
Regulus and Dumbledore were the only two wizards who figured out the Dark Lord’s secret to immortality on their own.
To the Dark Lord
I now I will be dead long before you read this but I want you to know that it was I who
dicovered your secret. I have stolen the real Horcrux and intend to destroy it as soon as I
I face death in the hope that when you meet your match you will be mortal once more.
The real locket had been hidden at Grimmauld Place when Harry and the Weasleys were staying there. In fact, several Order members tried to open it, but it was "stuck" (they thought). Hermione is the one who puts two-and-two together about that:
“Whether he’d manage to destroy it or not, he’d want to keep it hidden from Voldemort, wouldn’t he? Remember all those awful things we had to get rid of when we were here last time? That clock that shot bolts at everyone and those old robes that tried to strangle Ron; Regulus might have put them there to protect the locket’s hiding place, even though we didn’t realize it at . . . at . . . “
“There was a locket . . . In the cabinet in the drawing room. Nobody could open it." (DH10)
And indeed, the locket had been there until Mundungus Fletcher stole it and took it to Diagon Alley, where Dolores Umbridge confiscated it from him (DH11).
Harry let Kreacher keep the fake locket from the House of Black, and he wore it at the Battle of Hogwarts (DH36).
Family in general: "The Noble and Most Ancient House of Black" dating to at least the Middle Ages (OP6); descendent of Phineas Nigellus Black, a Hogwarts Headmaster. Parents convinced that to be a 'Black' made you "practically royal." Black family members tend to be named for constellations.
Mother: Walburga Black (1925-1985); her portrait is permanently mounted in the hallway of the Black family home.
Father: Orion Black (1929-1979), who heavily fortified the family home at 12 Grimmauld Place, London.
Grandparents: Arcturus (1901-1991) and Melania (McMillan) Black, and Pollux (1912-1990) and Irma (Crabbe) Black. Both sets of Sirius's grandparents were direct descendents of Phineas Nigellus Black. One of his grandfathers (either Arcturus or Pollux) was awarded Order of Merlin 1st Class for "Services to the Ministry" (OP6).
Aunts, Uncles: Most notably Uncle Alphard, who left Sirius money, and Aunt Elladora who started the tradition of beheading house-elves when they were too old to carry tea trays. (See the Black Family Tree for more).
Cousins: Narcissa (Black) Malfoy, Andromeda (Black) Tonks, Bellatrix (Black) Lestrange. Arthur Weasley is a second cousin once removed. Distantly related to Molly Weasley.
Family motto: Toujours pur (French for "always pure") (OP6).
Family crest: is mentioned in OP6 but not described; however, Rowling drew it on her Black Family Tree. It shows a shield with hounds rampant emblazoned with a chevron, 2 five-pointed stars and a short sword.
Siblings: Sirius Black (c.1960-1996).
Location of childhood home: 12 Grimmauld Place, London.
Hogwarts: Slytherin c.1972; Seeker.
Played Quidditch as Slytherin Seeker (DH10).
Other canon notes and references
Jo tells us "Regulus got in a little too deep. Like Draco. He was attracted to [Voldemort's power], but the reality of what it meant was way too much to handle." (TLC)
Regulus = Latin for 'the little king;' the brightest star in the constellation Leo. Regulus was also the family name of Marcus Atilius Regulus, a Roman naval commander who was legendary for his heroic self- sacrifice (CE).
In alchemy, a "star regulus" is formed from heating the shiny metal antimony, part of the process in creating the Philosopher's Stone.
Arcturus = Latin for 'bear guardian;' the fourth brightest star in the night sky, whose rising and setting was thought by the Romans to foretell tempestuous weather.
One of the tragedies in the story of Regulus is how little his brother Sirius thought of him. For instance, Sirius spread the story for years that he was "stupid" and a coward as a Death Eater, someone who would never even be noticed by Voldemort (Lupin, for one, believed the story). In some ways this is just basic sibling rivalry - the "Mom liked you the best" syndrome -- since Regulus was supposedly more obedient to their pureblood parents. But that is hardly fair since by his own admission, Sirius was always treated like royalty at home and everywhere in the Wizarding World as a member of the Black family.
From an early age the brothers were quite different -- rebellious Sirius with his Muggle pin-up girls, and Regulus with a "keep out" sign on the door that reminded Harry of Percy Weasley. When Sirius met James on the train to Hogwarts, he suddenly wanted to be sorted into Gryffindor, which must have disappointed his parents. When Regulus was sorted into Slytherin in the family tradition, suddenly there was House rivalry in addition to sibling rivalry.
Sirius found a new family as part of the Marauders, and spent more time on breaks away with James and his parents. When he was 17 he left home to get his own place, thanks to an inheritance from Uncle Alphard. This action supposedly broke his mother's heart, according to Kreacher the house-elf.
Due to all his absences from Grimmauld Place, Sirius just didn't seem to know Regulus very well because he was more concerned with fighting for the Order of the Phoenix. It just never occurred to him that Regulus might have been just as brave and heroic as he and James in standing up to Voldemort. Would Sirius have sacrificed himself in a far-off cave, so that no one ever knew about it? For James and Lily, he might have, yes. For the honor of Kreacher? It's hard to imagine. Sirius hated Kreacher, joked about his death, and even threw him out of the house. Regulus on the other hand was the "defender of house-elves," as Kreacher shouted at the Battle of Hogwarts.
From the Web
"Narcissa Malfoy and Regulus Black and the Downfall of Voldemort" from Mugglenet