Harry Potter: How Regulus Black Discovered Voldemort's Horcrux Secret
Regulus Black, a member of the infamous House of Black pure-blood wizarding family, was one of the first characters in the Harry Potter series to discover Voldemort's Horcrux secret. Regulus does not appear physically in any of the films but is mentioned throughout by various characters. His primary references came from his house-elf, Kreacher, who revealed that Regulus retrieved Slytherin's Locket after discovering it was a Horcrux and attempted to destroy it. This leads to questions of how he learned of Voldemort's Horcruxes in the first place.
The Black family featured a host of notable wizards, including Sirius, Regulus's older brother and godfather to Harry Potter. The family was one of the oldest and wealthiest families in the Wizarding World, also holding membership in the Sacred 28. Another familial trait was that most of the Black family were sorted into House Slytherin, with many being supporters of Voldemort, even a few becoming Death Eaters like Regulus. While Regulus didn't play as significant a role as many of his kin, he can ultimately be considered one of Slytherin's greatest heroes in the Harry Potter franchise.
Related: Why Lord Voldemort Didn't Know Harry Potter Was A Horcrux
In a 2007 interview (via Accio Quote/Bloomsbury), J.K. Rowling partially addressed how Regulus was able to deduce that Slytherin's Locket was indeed a Horcrux. She reiterated that Voldemort did not invent the magic behind Horcruxes, just that he was the first to make six of them. Immortality made him even more arrogant, dropping hints to others as he did before Harry in the graveyard at Little Hangleton. Rowling mentions that Voldemort also did this in front of Regulus, who was able to correctly guess why the Dark Lord was so
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