It’s perfectly understandable if fans need a refresher course on Regulus Black; as noted, he plays a very small role in the overall story of “Harry Potter.” Regulus is Sirius’ older brother who makes his pureblood parents proud by joining the Dark Lord Voldemort (Ralph Fiennes) and his Death Eaters as they rise to power … which disgusts Sirius, who fought against Voldemort with the Order of the Phoenix.
As it turns out, Regulus becomes disgusted with Voldemort’s monstrous ideas fairly quickly and sets out to defeat his master … by swapping out one of Voldemort’s Horcruxes for a fake and destroying the real one. Along with the Black family house elf Kreacher, Regulus infiltrates the secret cave where Voldemort kept a locket that held a piece of his soul, and the duo manages to remove the real Horcrux and replace it with a locket bearing Regulus’ initials: “R.A.B.” Tragically, Regulus was dragged underneath the cave’s water and killed by reanimated corpses known as Inferi, but Kreacher, a house-elf who had been ordered to return home, was physically compelled to follow the order and left without Regulus.
Sirius never learns any of this before he dies; it’s only Harry who discovers what Regulus was really up to. Still, it’s a pretty stunning story, and since it concerns Horcruxes, which must be destroyed before Voldemort can be defeated, it’s a pretty major part of the franchise’s climax.
