Regulus Arcturus Black's older brother had just returned from his first year at Hogwarts a few days ago. It had been an event that both excited and scared Regulus. Excited, as his brother was back, the brother he had always looked up to and admired, the brother that had always taken care of Regulus after Their Mother had taken out her madness on them, the brother that always made everything better. He was nervous however, as Mother had been especially angry when it turned out Sirius was in Gryffindor, and apparently friends with one of those 'filthy Potters', and Regulus didn't know what Mother would do to his brother.
Mother had proven Regulus's fears correct when the first word she had said when he'd stepped through the front door of 12 Grimmauld Place after being collected by Kreacher from the station was 'Crucio'.
She'd shoved a pocket watch in his hand, and Regulus had been forced to count out the long minute that she kept Sirius under the crucio. He made sure not to speed up the seconds, as he knew that if he did she'd just do it on him instead to 'teach him to count', and then would double it on Sirius. They had both learned the danger of not perfectly following their Mother’s orders the hard way.
Now Regulus was sitting on the bed in his room, leaning against the side wall, the only thing separating him from his brother, the brother he hadn't seen since Sirius had been thrown in his room and the door warded shut as soon as she was done crucioing him. And so it was there, moping about his brother that Kreacher found Regulus with a summons from his parents.
Regulus didn't even think about ignoring a summons. That would not be a good idea.
When he arrived down in the sitting room, his parents had already taken up their favourite seats in the two large comfortable armchairs that Regulus didn't ever dare sit in, leaving Regulus to sit on one of the multiple backless stools littered around the room. He ensured his body language was sufficiently cowed when he saw the look on his Mother's face.
"Your brother is a disappointment" was how she started, the insane anger they had come to expect from her bubbling through her calm facade.
"Not only is he a Black in Gryffindor, he has made friends with mudbloods and blood traitors." She continued. Regulus knew not to interrupt her.
"Your father and I have talked," and by that she means she had decided, Orion was a useless drunk, and anything important in the house was decided by Mother, "and we have come to the conclusion that it would be better to train you as heir."
What? Him as heir? What would that mean for him, would he have to undergo those lessons that always left Sirius shaking, what would that mean for Sirius, without the protective mantle of the heir around him?
"You have always wanted to be heir have you not?" Mother continued.
"Yes ma'am," he lied, throat raspy and dry, "I have always wanted to be your heir.”
Regulus slowly slumped forwards out of his seat as he felt his nerves light on fire. The pain rocketed through him as every single sensory neuron devoted to the task screamed 'agony'. He felt the cruciatus flood over him like fire and ice, his senses overstimulated in all the worst ways. He only noticed he'd lost control of his limbs when he kicked something in his spasms, and felt the blessed relief of slightly less pain where the injury distracted him slightly from the pain of the cruciatus.
"You should know better than to lie to me, you were always the weaker brother, your brother might be a disappointment, but at least he wasn't spineless." Mother spat, ending the cruciatus curse. "Get up." she commanded.
Regulus dragged himself off the floor, standing tall, hands clasped behind his back to disguise the quiver. "Good, at least you know how to act." She continued, before stopping for a moment, a sly smile spreading across her face.
"You still love your brother, the disappointment, do you not?" She asked, in a self satisfied drawl.
"Yes ma'am." He responded, too scared to lie.
The terrifying smile finished spreading across her face, "Good, then come with me." She said, standing up and beckoning Regulus to follow as she headed to the entrance hall.
As Regulus followed behind Mother, head bowed, they went up the main set of stairs, getting off on… Regulus and Sirius's floor? She walked straight over to Sirius's door, and after quickly checking behind her to see that Regulus had followed, she waved her wand to remove the complex wards surrounding it, and slammed it open.
Sirius jumped up at the sudden intrusion, from where he was lying on his bed reading some letters, and grabbed his wand, but Mother was already standing and had many more years experience, and was able to cast first.
"Imperio"
Sirius froze, his hands going to his side as his eyes went cloudy, and Regulus watched on in fear. Mother at that point stepped out of the way and dragged Regulus forwards so that he was looking straight at his imperiused brother.
"Now," Mother said, "Sirius, repeat after me, mudbloods are filth and deserve to die."
"Mudbloods are filth and deserve to die." Sirius intoned dispassionately.
"Again, repeat after me," Mother continued, "I am a disappointment to house black."
"I am a disappointment to house black." Sirius continued in his monotonous robotic tone.
"If I told you to floo over to potter manor, and murder James Potter, what would you do?" Mother asked
"I would floo over to potter manor and murder James Potter ma'am."
At that Mother turned to Regulus. "You understand your duty now?"
He did. He would have to be the heir, he would have to be perfect, he would have to be the mouthpiece for black family ideals that Mother wanted, or she would Imperius Sirius and force him to be the heir.
As he verbalised his understanding, she smiled that terrifying smile yet again, before cancelling the imperius, obliviating Sirius, and shepherding Regulus out of the room. She dismissed him as she was re-warding the door, which he was thankful for, as it meant that he was able to retreat to his room before the tears escaped.
He lay there, crying on his bed for a while, crying for who he had been, crying for what he would have to do, and crying for the love for his brother that he knew he would have to pretend to hate. When it was finally dinner time, he made sure that he was the perfect image of a pureblood prince, familial ring on his finger, wearing fancy robes with the black family crest engraved upon it, and a face that didn’t even have a trace of tears. As Regulus slowly descended the stairs to the dining room after being summoned by Kreacher, he carefully occluded away all the feelings of love that he still harboured for his brother, and made sure his expression was as regal as possible.
He barely even flinched when he saw his brother, an immense amount of pain hidden away so carefully that only Regulus would be able to recognise it. He pulled his face into a carefully crafted sneer, partly to hide the flinch, but mostly to play the act for their parents. “Mother, may I ask why the blood traitor has been allowed to join us?” Regulus looked over at Mother while he asked his question, but wasn’t quite able to avoid watching the crestfallen and betrayed look wash over his brother’s face out of the corner of his eye.
“I thought to see if he was even somewhat redeemable, but as he seems completely unwilling to repent for his Griffindorish ways, I fear that Potter boy has been a worse influence than we suspected.” Mother replied, a slight smile on her face.
Regulus threw Sirius another carefully crafted sneer, before walking around the table and sitting as far away from his brother as he could, which was next to Mother. Sirius stared at him for several minutes, confusion obvious on his face at just how different Regulus was acting to the last time they had talked. “Reggie, what are you doing? You’re supposed to be my brother, my best friend?” He eventually got up the courage to ask, confusion at his brother’s actions obviously overriding his fear of Mother.
Regulus recrafted the sneer on his face before replying. “That was before you betrayed our family, and replaced me, your true brother, with a Potter.” He spat the last word like a curse. “And don’t call me ‘Reggie’, that infantile nickname is unworthy of a son of house Black.” He turned back to Mother before continuing. “He really is a disappointment isn’t he?” Regulus asked.
The slight smile on Mother’s face was almost identical to the one she’d been wearing earlier. “Yes, his speech has deteriorated in that tower of idiots, hasn’t it?” It was a rhetorical question, but Regulus made sure to nod and act as if he agreed with Mother, it was never a good idea to make her think you disagree with her. “Well then, off to bed with you, you foul mouthed idiot, and don’t even dare to curse our dining table with your idiocy unless specifically summoned.” As Sirius left the room, he threw back one last hopeful and confused glance at Regulus, but he only responded with one last disapproving look before turning back to engage their parents in polite conversation.
And so Regulus made his choice, he would play the part of the perfect Black heir to protect his brother, and would make any sacrifices needed to protect the one he cared about the most.
And so when Sirius got back from his second year at hogwarts, and the same procedure as last year of crucioing him and then locking him in his room occurred, it was Regulus that wielded the wand.
And when shopping for Hogwarts later that summer, Regulus and Mother reached an unspoken agreement never to talk about how his wand was pear wood with a unicorn hair core.
When Regulus sat on the old stool in front of the entirety of Hogwarts, he argued and argued, and finally got the hat to agree to put him in Slytherin instead of Hufflepuff.
Whenever Regulus passed his brother in the halls, he made sure to ignore him, and if that wasn't possible, respond only in hexes and jinxes.
And when he saw the boy sitting next to his brother across the hall, and realised he was falling in love with James Potter, he didn't act on it, and just used James's love of Lily to help fuel his act of muggleborn hatred.
But he never forgot that it was Sirius that he was doing everything for.
And so Regulus always did his best to subtly direct his fellow Slytherin's more violent urges away from the marauders.
So when he figured out in his second year that his brother's boyfriend was a werewolf, a secret that would wreck his brother if it came out, he didn't tell a soul.
And when he figured out his brother was planning to run away in the summer after Regulus's third and Sirius's fifth year, he snuck out of his room in the dead of the night, took Mother's wand from her room, and used it to unblock the floo.
He only broke his silence about his secrets once, at the start of his fourth, and Sirius's sixth, year. He went to Dumbledore, explained that Sirius was in danger due to being disinherited, explained everything that had occured, including Mother's penchant for the cruciatus curse, and her Imperius threats, and kneeled on the floor of the headmaster's office and begged him to do something to protect his brother.
The headmaster only responded by assuring Regulus that Sirius was safe, patting him on the head, calling him brave, and assuring Regulus that "sometimes, one must make sacrifices for the greater good."
Regulus barely held it in for long enough to make it back to his dorm, and close and silence his curtains before collapsing into ugly tears. He knew he could never reveal his secrets to anyone again, if he'd been dismissed so out of hand by the 'leader of the light'.
And so he continued with his original plan, working hard to be the perfect pureblood heir, and joining the death eaters at 16, the youngest anyone had ever joined.
When he was marked, he did not crumble to his knees or cry out in pain like everyone else, he stood strong, happy that it was his arm being tainted by the inklike stain, and not his brother's.
When he faced Dumbledore's little force in death eater raids, he made sure to target Sirius's close friends with spells that would take them off the front lines for a long time, but wouldn't kill.
And when Kreacher returned from being borrowed by the Dark Lord, talking about dark artefacts that seemed the Dark Lord and pain water, Regulus knew what he would do.
He knew from what Kreacher had described of the traps, he wouldn't survive his attempt at redemption, so he made the preparations he needed to so that Sirius would be safe from Mother's wrath.
Flooing into his parents manor they were staying in out in the country to ride out the war, he cast two killing curses, the last he'd ever cast, and flooed out before their bodies even hit the floor.
Walking into Gringotts, he claimed the Black lordship, as he'd become the heir when Sirius had been disowned, and reinstated his brother before making sure that everything owned by the family estate would be left to him when Regulus died.
One last floo trip took him back to 12 Grimauld place, to pick up Kreacher, who apparated them to a rocky cliff by the sea.
One blood sacrifice, one tense boat ride, and a drink that tasted like distilled cruciatus later, he held in his hands a locket that he instantly recognised as a horcrux.
He was barely able to replace the horcrux with his fake, and hand the locket to Kreacher with instructions to destroy it, before the overwhelming thirst overtook him, and he dragged himself down to the infested water for a drink.
And as he was dragged down to the bottom of the lake by a host of inferni, Regulus was happy.
Because now, Mother was dead, and Voldemort was no longer immortal.
Sirius was safe.