J.K. Rowling revealed how Regulus Black discovered the Horcruxes of Voldemort secret. But key parts of The Deathly Hallows paint the whole picture.
Regulus Black, a pure-blood wizard from the famed House of Black, was one of the first persons in the Harry Potter book to uncover Horcruxes of Voldemort secret. Regulus does not see themself physically in any of the films. But he is mentioned by numerous people throughout.
His major references came from his house-elf, Kreacher, who stated that Regulus retrieved and attempted to destroy Slytherin’s Locket after realising it was a Horcrux. This raises the question of how he first became aware of the Horcruxes of Voldemort.
The Black family included a number of renowned wizards, including Sirius. Regulus’s older brother, and Harry Potter’s godfather. The family was one of the oldest and wealthiest in the Wizarding World. As well as a member of the Sacred 28.
Another familial feature was that the majority of the Black family belonged to House Slytherin, with many of them supporting Voldemort and even becoming Death Eaters like Regulus. While Regulus did not play as prominent a part as many of his contemporaries. He may be regarded as one of Slytherin’s greatest heroes in the Harry Potter universe.