Regulus Arcturus Black (R.A.B.) died by drowning in the Horcrux cave's lake after being dragged underwater by the Inferi. The Events Leading to His Death As detailed in the Wizarding World feature on unsung heroes, Regulus was a young Death Eater who defected after uncovering Lord Voldemort's horrific plans and witnessing the Dark Lord's cruel treatment of his family's house-elf, Kreacher. He pieced together that Voldemort had created a Horcrux-specifically Salazar Slytherin's locket-and vowed to destroy it. Drinking the Potion: Regulus ordered Kreacher to take him to the hidden seaside cave. To reach the Horcrux, Regulus drank the burning, mind-altering Emerald Potion himself to spare Kreacher from experiencing the agony a second time. The Fatal Mistake: The potion induced an unbearable, burning thirst. Driven by desperation, Regulus crawled to the edge of the dark lake to drink the cave's water. Dragged Under: Touching the water disturbed the Inferi-the army of reanimated corpses guarding the basin. They emerged from the dark depths, swarmed the weakened wizard, and dragged him to his death. The Final Order: Before he vanished beneath the surface, Regulus gave Kreacher a final command: replace the real Horcrux with a fake duplicate containing his famous note, flee the cave without him, find a way to destroy the real locket, and never tell his parents what happened.