"I know what you're going to say Evan, but I know...I already know. He had his foot in both camps, so no matter how the war ended, he would emerge on the winning very Slytherin. But this is not how my little brother will be remembered; some two timing scumbag. He will be celebrated as a hero, same as you."
Sirius Black talking to Evan Rosier about his brother's loyalties.
Regulus Arcturus Black, O.M. (First Class), (7 June 1961 - 31 October 1981) was a pure-blood wizard born to Orion and Walburga Black, and the younger brother of Sirius Black. He was a member of the House of Black; an old and respected wizarding family. He attended Hogwarts School of Witchcraft and Wizardry from 1972 to 1979, and, to nobody's surprise, was Sorted into Slytherin house.

Regulus became a Death Eater in his youth, however, he became disillusioned with Death Eater philosophy after witnessing Lord Voldemort's method of achieving his aims, and defected to the Order of the Phoenix; becoming a double agent, working closely with his best friend and Order spy, Evan Rosier. Thanks to his skill in Occlumency, Regulus was able to, with tremendous difficulty, keep his true allegiance a secret from Voldemort. During this time, Regulus fathered a child, Dean, with his Muggle wife, Jennifer. As the war heated up, Regulus abandoned them in order to keep them safe, lest the Death Eaters find out about them.

After an unfortunate blunder on the part of Evan Rosier, Regulus took the opportunity to out him as a spy to get closer to Voldemort. He successfully infiltrated the Death Eaters at the highest level after killing Dumbledore (on Voldemort's orders, but at Dumbledore's insistence) during the Death Eater coup of the Ministry, and was made his right-hand man. During the Battle of Hogwarts on Halloween, 1981, Regulus showed where his loyalties lay in an act of heroism, rescuing his brother from death at the hands of Death Eaters. Unfortunately, he was severely punished afterward, and Lord Voldemort had his familiar, Nagini, bite Regulus, cursing him with a slow death, finally passing on in the arms of his brother; Sirius.

His death was not in vain, as Regulus' actions resulted in the successful defeat of Lord Voldemort and the surrender of his Death Eaters at the hands of Severus Snape. His true loyalties were made known to the public, and Regulus was posthumously awarded the Order of Merlin, First Class, and celebrated as a hero. He would have two nephews through his brother Sirius; Severus and Perseus Black, the latter's middle name being his namesake.


British Wizarding Civil War

Fall of the Ministry, death of Dumbledore and Battle of Hogwarts

Regulus: "Absolutely not! I am cashing out now, I will no longer serve you. The Order of the Phoenix is in shambles; you are the only thing that stops it from outright crumbling. Voldemort will take over the Ministry tomorrow. Face it, the light has lost."
Albus Dumbledore: "As of this moment, you would be correct. There is only one man who can end this war in favour of the light, and I believe my death will be the catalyst for his involvement."
Regulus: "Severus..."
Albus Dumbledore: "Precisely."
— Regulus' final true conversation with Dumbledore before the latter's death.

End of the war and legacy

"I no longer serve you..."
—Regulus' last words to his former master.