Chapter Text
Regulus hadn’t expected it, and that’s what hurt the most.
He had watched their brotherhood unfold in real time, unravelling before his eyes. The way they moved around each other with effortless ease, those sly smiles, those knowing glances, as if every moment between them was laced with an unspoken joke. He’d seen the way Sirius laughed with him, loud and unrestrained, the kind of laughter that shook shoulders and lit up faces. Sirius never laughed like that with Regulus.
Regulus had seen it all. Every look. Every grin. Every moment.
But he never expected it to come to this.
He hadn’t expected Sirius to leave.
Forever.
And it wasn’t just that Sirius had left. It was where he had gone.
He could have fought for them. Could have stayed and stood beside Regulus, like they always said they would when they were boys hiding under stiff silk sheets in the cold manor, whispering about escape routes and revolution. They had made promises in the dark, silent, childish oaths that they would face their parents’ madness together. That they would survive the name they’d been given. Together.
Instead, he left to him.
In hindsight, Regulus should have seen it coming. The signs were all there, scattered across conversations and silences, hidden in the defiant way Sirius held himself during their parents’ tirades, in the way he always looked out windows like he was searching for something just out of reach. But Regulus hadn’t seen it. Or maybe he hadn’t wanted to. His vision had been clouded by hope. Hope that his older brother would always have his back, that they would weather the storm of their parents together, that Sirius would always, always choose them.
Instead, he chose a boy he’d known for only six years.
James Fleamont Potter.
“I think they’re fucking,” Barty drawled, his voice far too delighted for Regulus’s liking. The words pulled Regulus sharply from his thoughts. He grimaced as they sank in and shot Barty a glare.
“Who, Potter and Black?” Evan asked, snickering as he speared a sausage with his fork.
“Who else?”
“Don’t be daft,” Evan muttered, rolling his eyes as he dropped a mountain of mashed potatoes onto his plate. “Have you seen the way Black looks at Lupin?”
Regulus didn’t say a word. Instead, he glanced past them.
The Great Hall buzzed around them, filled with the clatter of cutlery on ceramic, the low roar of hundreds of voices overlapping, laughter bouncing off the enchanted ceiling that mimicked the overcast sky above. At the far end, the Gryffindor table seemed a world away, though the laughter from it carried easily across the hall. Sirius and his little gang were laughing obnoxiously at something Regulus was certain wasn’t that funny. Sirius and Lupin sat side by side, far too close, heads nearly touching as they leaned together, lost in laughter that looked a little too effortless, a little too intimate.
“Maybe all three of them are fucking!” Barty offered, tone gleeful, as if he genuinely hoped it was true just to stir the pot. Regulus’s fingers tightened around his goblet, and for a moment, he considered throwing the water straight at his friend’s face.
“Can you not discuss my brother’s sexual proclivities for five minutes and just eat?” he hissed, his lip curling into a sneer.
Barty held up his hands in mock surrender. “As you wish, Reg. But you’re the one obsessed with Potter and Sirius’s little love story.” He bit into a chicken drumstick as if to punctuate the thought.
“Yeah, and why do you think that is, smartass?” Evan muttered, elbowing him. Then he turned to Regulus, his expression shifting into something darker. “If I were you, I’d do more than just obsess. I can’t believe he just left. Fucking coward.” He spat the word with venom and shook his head. “Some Gryffindor, he is.”
Regulus didn’t say anything. Didn’t say that Sirius was every bit courageous to leave. That Regulus was the coward for never letting himself think that leaving was even an option.
“Oh yeah?” Barty turned to Evan, lips curled in a taunting smirk. “And what would you do, Ev?”
Evan didn’t even pause to think. “Hex him. And make him feel every single bit of pain Regulus is now going to have to endure alone in that house.”
There was a pause, long enough for the scrape of a fork to echo off a nearby plate.
Barty made a face, exaggerated and unimpressed. “God, you’re so boring. And honestly, a little tasteless. Violence is so overdone.”
Evan rolled his eyes. “Not everything has to be some twisted performance, Barty.”
“But it should be,” Barty said, grinning like a cat with blood on its paws. He turned to Regulus, who had barely moved, his gaze still half fixed on the Gryffindor table, his expression unreadable. “Reg? You listening?”
Regulus gave a vague, distracted hum. It could’ve meant anything. Barty took it as permission.
“If I were you,” he began, dramatically, drawing out the words like a spell, “I’d seduce James Potter.”
Regulus blinked, finally dragging his eyes away from Sirius and his golden circle of friends.
“I’m sorry—what?” he asked flatly.
“You heard me,” Barty said, all smug delight and glinting eyes. “I’d try my absolute hardest to make Potter fall in love with me. Really reel him in. Make him laugh. Make him blush. Make him obsessed. And once he’s in so deep, he can’t hide it, once it’s so obvious even blind, emotionally-stunted Gryffindors could see it... that’s when Sirius finds out.”
Evan snorted into his pumpkin juice. “You’re insane, Barty.”
Barty ignored him, eyes still on Regulus like he was delivering some great, theatrical truth. “Picture it: Sirius Black losing his mind because his precious best friend—his golden boy—is secretly boning his little brother behind his back. It would be glorious.”
Regulus stared at him. For a long moment, he said nothing.
Then, simply, “You’re disgusting, you know that.”
Barty bowed, mock-graciously. “And yet, you are listening now, aren’t you?”
Regulus didn’t say anything. But the words lingered. He almost laughed at the ridiculousness of it. Almost. But instead, the silence that followed Barty’s theatrics settled heavily in his chest, tight, cold, and unshakable.
He glanced again toward the Gryffindor table. Sirius was still smiling, still shining like he always did when he was with them. His laughter, careless and open, floated across the hall like it had no right to.
And there was Potter. Right beside him. Always.
Regulus still remembered the first time he saw him. That first train ride six years ago now, when Sirius had dragged him into their compartment like it was the most natural thing in the world. Introduced him to Potter with pride in his voice and a hand on his shoulder. Regulus had spent the entire journey beside them, quietly content to listen, to exist in their warmth for a little while.
And then came the Sorting.
The Sorting Hat had barely touched his head before it shouted, “Slytherin!”
And just like that, something shifted. Not loudly. Not dramatically. But it was there.
Potter didn’t meet his eyes when Regulus passed their table. Sirius’s smile flickered like a candle in the wind. And from that night on, it was like Regulus had been pushed to the edge of a frame he used to be in the centre of.
After that, it was always Potter. Always Sirius and James Potter.
Six years, and their friendship had only grown stronger, louder, more blinding. Like the rest of the world had faded around them.
Like Regulus had faded.
There was no space left for Regulus after that. Not really. Not in Sirius’s world. Not where Potter lived. Regulus had spent years trying to understand what changed. When their brotherhood shifted into something that only went one way. When Sirius stopped looking at him like he was something worth protecting. When Potter started smiling and Sirius forgot that Regulus was still standing there, just a few steps behind, waiting.
Waiting to be chosen.
He wasn’t.
And now, sitting at the Slytherin table, flanked by two boys who only knew how to mock or destroy, Regulus realized Barty’s idea, insane as it was, held a certain appeal.
Not because he wanted James Potter. He didn’t.
But because Sirius did.
If Sirius could laugh that easily now, if he could walk away and never look back, then maybe he deserved to lose something, too.
Maybe Regulus deserved to be the reason.
To ruin the one thing Sirius had clung to, the one bond stronger than blood. To make Potter look at him in a way Sirius would never want. Just to see what it would do to his brother. Just to see him crack.
Seduce him. Make him fall. And when he does, shatter everything they built.
It would be cruel. It would be calculated.
It would be deserved.
“Reg?” Barty’s voice broke through his thoughts again, too chipper for the weight of the moment. “You're awfully quiet. I think you like the idea.”
Regulus didn’t answer. He reached for his goblet instead, sipping the water like it might cool something burning deep inside his chest.
He wasn’t sure he’d ever act on it.
But for the first time in weeks, the thought of doing something, anything, felt less like grief and more like power.
And if James Potter was the price of that power...
Sirius made his choice six years ago.
Maybe it was time Regulus made his.
“Yeah, great plan, Barty,” Evan drawled, dragging his fork through his mashed potatoes like they’d personally offended him. “Have Regulus seduce the straightest Gryffindor there’s ever been, who also, might I add, has been egregiously in love with a certain Lily Evans for the past six years.”
Barty didn’t flinch. He turned to Evan, unimpressed. “Details. Minor ones.”
“Oh, minor, is it?” Evan scoffed. “The fact that he’s spent half of his time at Hogwarts pining over a girl who thinks he’s a toe fungus, that’s minor?”
Barty leaned sideways lazily, stretching one arm on the free space on the bench. “Please. Potter’s the kind of boy who could fall in love with a lamppost if it flashed him a charming enough smile. He’s emotional. Predictable. Easily manipulated. And most importantly...” He flashed a wolfish grin, “A Gryffindor.”
Evan’s brow furrowed. “And what’s that supposed to mean?”
Barty let out a quiet, almost amused sigh, glancing toward Regulus as if explaining a simple truth. “It means Potter’s the sort who’ll try to believe the best in people, no matter how much evidence there is to the contrary. He’s trusting to a fault. Loyalty runs in his veins like a sickness and he’ll stick to his principles even if it means ruining his own skin for it. And that…” He smirked, leaning forward slightly, “makes him so much easier to play with.”
Regulus stared into his goblet, swirling the water absently. He wasn’t sure what was more unsettling, that Barty was speaking so flippantly about the whole thing, or that none of it sounded entirely untrue.
“And how is it that you know so much about him, Barty?” Evan asked suspiciously, narrowing his eyes in question.
“I’m an observer, what can I say?” He shrugged. “That aside, listen.” Barty added, shooting Regulus a look, “It’s not about making Potter fall in love forever. Just long enough for Sirius to notice. For him to crack. That’s the goal, isn’t it?”
Evan gave Regulus a sideways glance, wary now. “You’re not actually considering this, are you?”
Regulus didn’t answer.
He didn’t need to.
The idea had already begun to take shape, quiet and poisonous, threading itself through the jagged spaces Sirius had left behind. Regulus sat silently, letting the conversation wash over him. It wasn’t just the revenge that fascinated him. It was the idea—James Potter, so sure of his own righteousness, so wrapped in the belief that everyone was as loyal and good as he was. Regulus couldn’t decide if it was laughable or tragic. But this plan brought out feelings within him an anger he didn’t think was ever possible to bury.
And it wasn’t just anger, if he was honest with himself. It was something uglier. Something more tangled and bitter.
Jealousy.
He hated James Potter. Not because of his too-healthy upbringing, not because of his arrogance or the way he strutted down the corridors like he owned the castle. Those were just convenient excuses. They gave the jealousy something polite to wear.
The real reason?
Because Potter had taken something that had once been his.
Because Regulus had Sirius first.
He was the one who had huddled beside him in their shared bedroom when the shouting downstairs grew too loud. He was the one Sirius used to sneak sweets to under the dinner table, who he used to protect from their mother’s sharp tongue and crueller silences. He was the one Sirius promised, in whispers and midnight oaths, that they'd always have each other.
And then, one day, he didn’t.
Sirius had found someone else to belong to. Someone who didn’t carry the same name. Someone who didn’t come with strings. Someone who made it easy to forget everything Sirius was supposed to stay and fight for.
He had chosen James’s house. James’s friendship. James’s world.
Forever.
And Regulus, left standing in the ruins of that choice, had never gotten the chance to ask why.
What had Potter done that Regulus hadn’t?
What had he offered, besides warmth and jokes and a place in a dormitory lit by golden light and hollow words like bravery and honour?
Regulus had needed Sirius. Had needed him.
But James had wanted him. And that, apparently, had been enough.
It was never just about love or loyalty. It was about being chosen.
And Regulus hadn’t been.
Not by Sirius.
Not ever.
“Why not?” Regulus said coolly, his voice low but clear as he stared straight into Barty’s crazed, gleaming eyes. “What have I to lose?”
“The last shred of whatever relationship you still have with your brother,” Evan shot back, his tone surprisingly firm, eyes narrowing with something dangerously close to concern. “Think about it, Reg. If this actually works, if you go through with this, Sirius will never speak to you again.”
Regulus didn’t flinch. His fingers marched against the wooden table, calm and steady. “He’s already gone, Evan.”
“That’s not the same,” Evan insisted. “Distance is one thing. Cutting the cord entirely is another.”
Regulus turned to him then, expression unreadable, but something dark stirred behind his eyes. “He cut the cord when he walked out that door. When he left me to rot in that house alone. When he chose Potter, and Gryffindor, and some fantasy of a better life, and didn’t once think to look back.”
The words came bitter and sharp, each one honed over years of silence and disappointment.
“If he cared about what was left of our relationship,” Regulus continued, voice like frost, “he never would’ve left in the first place.”
Barty gave a soft whistle under his breath. Evan sat back, jaw tightening, unsure of how to argue with that kind of hurt.
“This isn’t about some pathetic attempt to win him back,” Regulus said. “I’m not trying to repair anything.”
He looked back toward the Gryffindor table, to the place where Sirius and James sat like they were the centre of the universe.
“This is about ruining them.”
Barty let out a slow grin, something wicked flickering across his face. “Now that’s the spirit.”
Regulus didn’t smile. There was no pleasure in this. Only precision. Only purpose.
Let Potter lose what he so carelessly stole. Let him feel the fracture. The betrayal. The rage of being left behind.
Let him finally understand what it meant to be replaced.