Sirius's younger brother. As a young man he was extremely proud of who he was and of who his family was, and he joined the Death Eaters because he truly believed in the cause. Despite these questionable views, Kreacher explains that Regulus was extremely kind to him—though this assessment is complicated by the revelation that Regulus volunteered Kreacher to help Voldemort by testing the potion protecting his locket Horcrux, a potion that makes the drinker relive horrific events from their past. Regulus served Voldemort until he decided to work against Voldemort from within by stealing the locket. According to Sirius, Regulus got cold feet and tried to abandon the Death Eaters but, due to his lack of importance within the organization, he was killed by one of Voldemort's followers and not Voldemort himself. Voldemort presumably never discovers that Regulus was working against him.
Regulus Arcturus Black Character Timeline in Harry Potter and the Deathly Hallows
The timeline below shows where the character Regulus Arcturus Black appears in Harry Potter and the Deathly Hallows. The colored dots and icons indicate which themes are associated with that appearance.
...Kreacher says he saw Mundungus steal the locket. Harry asks Kreacher to tell them what Regulus has to do with the locket. Kreacher explains that Regulus joined the Death Eaters and,... (full context)