Muggles' Guide to Harry Potter/Characters/Regulus Black

Marriage status Unmarried

Overview
Regulus Black is the brother of Sirius Black. He is also a former Death Eater, although he defied Voldemort and found, and intended to destroy, Salazar Slytherin's locket (a horcrux) and died when dragged into the lake and drowned by Voldemort's inferi.

Order of the Phoenix
Mentioned in passing only; discussing the tapestry of the Ancient and Honorable House of Black on the parlor wall at 12 Grimmauld Place, Sirius mentions his brother Regulus, and that he is believed to be dead. As mentioned, he was taken in by the Death Eater idea, and was apparently fairly high in the ranks when he came to his senses and tried to quit. Sirius dismissed him as not being important enough for Lord Voldemort to have killed him personally, instead believing that he was simply ordered to be killed.

Later on, shortly after Harry sees the vision of Sirius getting tortured, Ron suggests that "Sirius' brother" (meaning Regulus) could have told Sirius how to retrieve the "weapon" that Voldemort has apparently been hunting, since Regulus had been a Death Eater.

Half-Blood Prince
He is mentioned a few times early on. First Horace Slughorn mentions that Regulus was in his house while he was a teacher. Later Remus Lupin tells Harry that Regulus only managed to live a few days after trying to leave the Death Eaters.

When Harry and Professor Dumbledore go on a mission to recover a supposed Horcrux of Voldemort's, it turns out to have been already replaced by a locket. This locket contains a note signed by one "R.A.B.". Many readers believe this signature to be Regulus'.

Deathly Hallows
On the day after their return to Number 12, Grimmauld Place, Harry sees the sign on Regulus' bedroom door and discovers that Regulus' middle name is Arcturus. From this, he concludes that Regulus is the one who left the note, signed R.A.B., in the locket Harry and Dumbledore found in the cave. While Harry, Ron, and Hermione immediately search for the locket, they discover nothing. Hermione remembers that they had found a locket when trying to clean up the house, and that Kreacher had a habit of stealing back those Black family relics he considered valuable. When Kreacher's lair proves empty, Harry summons Kreacher, who eventually tells them about the locket.

Voldemort had demanded a house elf, and Regulus had volunteered Kreacher. The Dark Lord had taken Kreacher to an island on a lake, and had forced him to drink a potion that made him remember horrible things. The Dark Lord had put a locket in the bowl where the potion was, had refilled the bowl, and then had left Kreacher there. Kreacher had crawled to the edge of the lake to drink, had been seized by Inferi, and been dragged underwater. He had then returned to his master, Regulus, as ordered.

On hearing what the Dark Lord had done, Regulus had ordered Kreacher to take him to the place. He had then ordered Kreacher to force him to drink the potion in the basin, then take the locket, replace it with one he would provide, and destroy the locket. Regulus had drunk all the potion, and had then crawled to the waters' edge to drink, and had been dragged under by Inferi. Kreacher had never seen Regulus again, and he was distraught that, despite having managed to put the replacement locket in the basin, he had been unable to destroy the locket from the basin, and that the locket had been stolen from him by Mundungus Fletcher.

Harry wins Kreacher over by telling him that they, too, have the mission of destroying the locket, and by giving him the locket that Regulus had told Kreacher to put in the basin. Overjoyed at having this artifact of his late master returned to him, Kreacher heads off to find Mundungus and bring him back to Harry for questioning.

Kreacher leads the other house-elves into battle telling them to fight in the name of Harry Potter and Regulus Black.

Strengths
Regulus must have been relatively skilled in Occlumency  to have blocked Voldemort from reading his thoughts regarding the Dark Lord's evil plans. He also likely knew a lot about the dark arts seeing as he knew about Horcruxes.

Regulus showed courage and integrity when, finally rejecting Voldemort's beliefs, he attempted to destroy Voldemort's Horcrux that was hidden in the sea cave, even knowing he would never survive. Although he intended for Kreacher, his House-elf, to complete the task, his plan failed. However, his actions later allowed Harry to find and destroy the real Horcrux.

Weaknesses
Like his family, Regulus was easily swayed by Lord Voldemort's "pure-blood" rhetoric. He learned too late that the Lord Voldemort's beliefs were far more evil than he had ever realized and being a Death Eater involved far more unpleasantness than he was capable of. Once he entered the Dark Lord's service, however, he was unable to escape. Sirius was his brother and he loved him and couldn’t be with him.

Relationships with Other Characters
Brought up in a rabidly Pureblood family, Regulus found that Voldemort's blood purity ideas resonated with him, and he became a Death Eater during Voldemort's first rise to power. His loyalty to Voldemort seems to have waned when he discovered how Voldemort put his ideas into practice, and he turned against Voldemort with his last living acts.

Sirius tells us that Regulus was his mother's favorite, as, in so far as she knew, he had stayed true to the ideas of blood purity that she had espoused.

Analysis
It is interesting how much we can see about Regulus without ever actually meeting him. Initially, at any rate, Regulus' motives remain unclear; Sirius tells us that he had gotten relatively deep into Voldemort's organization, then, they believe, gotten scared and tried to get out, and been personally murdered by Voldemort for his troubles. As it turns out, his disillusionment with Voldemort's organization was probably growing for quite some time, as when he did choose to act, it was a fairly major stroke against Voldemort. By that time, Regulus must have been reasonably close to Voldemort's inner councils, as he had been aware that Voldemort wanted a house elf, so he would likely have been within range of Voldemort's legilimency. Because of that, he would have had to protect his mind from inspection as his disillusionment grew.

In the end, his sacrifice was apparently futile, as Kreacher was never able to destroy the locket he carried away, and as Voldemort had, at that time, possibly as many as four additional Horcruxes hidden away. The Horcrux within the locket is, of course, eventually found and destroyed. Kreacher might have been able to destroy it if Regulus had told him what it was, but Kreacher apparently was not told that it was a Horcrux he was trying to destroy, though Regulus' note indicates that he knew what it was.