Regulus Black

Regulus Black 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 travelled 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 can.
I face death in the hope that when you meet your match you will be mortal once more.
R.A.B. (HBP29)

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).