A/N: This is based largely on a Marauder's Era RP I'm in, where I play Regulus (and James, and Narcissa, and... XD) A lot of the backstory mentioned in this fic comes from that RP, not book canon, so if you don't recognise anything, that's probably why.
Dedicated to Spider, because a) she plays an awesome Sirius, b) she inspired this story, and c) she is just generally concentrated awesomesauce.
As always, concrit is loved. Enjoy!

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The laughter had not yet died from his face, but his eyes widened in shock as Bellatrix's curse hit him squarely in the chest. There was pain, swelling and howling, and the sound of his own blood, roaring in his ears. Briefly, he was aware that he was falling.

Then, nothing.

He awoke into darkness so complete that it took him a moment to realise his eyes were open at all. It was silent, utterly, terrifyingly silent, and when he stood, he couldn't feel the ground beneath his feet. He was standing, nonetheless, and, since he was standing, the choice seemed between walking and sitting back down and waiting to be rescued. The latter seemed… unlikely, at best, and Sirius Black had never been one to sit still when he could avoid it.

Besides, this place was too like Azkaban. Too close. Too imprisoning. He had to find the edges, if there were any, find out whether he was really trapped. Find a way out.

The pain had gone from his chest, which was something of a relief, but he couldn't remember why. The thoughts – the memories – were fuzzy and unclear, as though he had been drunk.

There had been a boy there - Harry, he told himself sharply. The boy was Harry. His godson. James' son. And the woman… Bellatrix. She had cursed him, and…

And what? He couldn't remember. No matter how hard he strained to think of it, he couldn't remember.

Tears starting to his grey eyes, which he wiped hastily with the back of his hand, he put one arm out in front of himself, groping at the air. When his fingers failed to stub against any obstacles, he started walking, still feeling around blindly for anything in his way. It was a strange feeling, walking through such total nothingness, and he had no idea how long he could possibly have been walking when he saw the light.

It took him a moment to register its existence; at first, he took it for a figment of his imagination. But when it failed to vanish as he grew closer, and, in fact, only got brighter, hope began to rise in him. He smiled slightly, hurrying his pace to a kind of shambling run, although he kept his hands out in front of him, still unable to see.

When he saw who was sitting there, though, he stopped in his tracks, his jaw dropping.

The dark-haired young man who sat in the light could almost have been his reflection, less eighteen years of hardship. He was tall, slender, and good-looking, and Sirius recognised him at once.

How could he not, when that face haunted him?

At that moment, the young man looked up from the chessboard he was studying, glancing into the darkness distractedly. He obviously hadn't expected to see anybody there, and when he did, his eyes widened. Still, he didn't seem to recognise Sirius, and Sirius wasn't surprised. After all, it had been a long, long time since they had seen each other, and he had been another man. They both had.

"Hello, Regulus." Blinking as the brightness dazzled him, he stepped into the full light.

The young man blinked back at him for a moment, uncomprehending, then his face cleared, and he smiled. It was a small, thin little smile, almost shy, but it was genuine.

"Sirius?" he asked, quietly, as though he didn't quite dare to believe it. "What… what happened."

"Azkaban happened," Sirius said bitterly, collapsing in the chair opposite Regulus and looking moodily at the chessboard. "Twelve years of Azkaban, thanks to your Death Eater mates."

Regulus bit his lip. "Sirius… shit, Sirius. I'm so sorry."

"Yeah, well." Sirius sighed, mollified a little. "My Marauder mates weren't exactly clean as the driven snow themselves." Thoughtfully, he picked up a white pawn from the board, toying with it. "Where are we?" he asked after a moment, looking up. "Am I dreaming this? Because this can't be real. You're dead."

Regulus nodded solemnly. "I know. So are you."

Every piece on the board shook as Sirius shoved himself violently to his feet, eyes wide. "I'm not dead!" he protested, but even as he did so, it began to slot into place in his mind. The veil… the curse… the tunnel and the light…

"I'm not dead," he repeated, but he sounded uncertain. Sinking back into the chair, he dropped his head into his hands. "I can't be."

Regulus watched him levelly, sighing. "Sirius, look. I know it's hard to accept, but…"

"Hard to accept? Is that what you think?" Sirius let out a bitter, barking laugh. "When your friend's betrayed you and your other friend's dead and you're in Azkaban, that's hard to accept! The only time you ever found anything hard to accept, you ran away, didn't you? You got in too far, and you ran away."

Insofar as it was possible, given that his skin was already alabaster white, Regulus had paled.

"Is that what you think?" he asked quietly, his voice shaking with emotion. "I thought you knew me a bit better than that, Sirius. Is that really what you think?"

Sirius blinked.

"Yeah," he said after a moment, putting the pawn back on the board with a sharp click. "Yeah, that's exactly what I think."

Regulus went very quiet for a moment. Then he bit his lip. "I turned my back on the Dark Lord because you were right," he said after a long moment. He seemed to be forcing the words out, from the way his voice trembled as he said them. "Because I saw what they were doing to people – to wizards as well as to Muggles. And I didn't like it. I wanted to change it." He sighed, picking up a black pawn and moving it a couple of squares across the board, towards where Sirius had put down the white pawn. "But I didn't see how I could, and I was scared and… Sirius? Do you remember the Christmas before you left home?"

Sirius looked up sharply. "We were… playing chess?" he said slowly, frowning.

Regulus nodded. "It was a long time ago for you, wasn't it? For me, it's only a couple of years ago, but that's still a while."

"You said the Hat gave you the same choice as me." Sirius looked down at the board, smiled slightly, and moved another pawn. "Gryffindor or Slytherin. I wasn't… I wasn't all that surprised. You said you picked Slytherin because you didn't want to upset the family."

"And…?" Regulus prompted, unsmiling. Looking down at the board, he considered the move Sirius had made.

He was still regarding the board thoughtfully when Sirius looked up at him, frowning, and said slowly, "And… because the Hat said that, in Slytherin, you could change the world. Is that why you joined the Death Eaters?"

Regulus nodded, moving his bishop across the board. "I thought I was doing the right thing," he said quietly, not looking up as he set the piece down softly. "I could change the world for the better, that's what the Hat said. So I tried to."

"And when you saw you were caught up in making it worse…" Sirius said, looking up at his little brother, who suddenly looked very young indeed. Sighing, he moved his knight. Without a moment's hesitation, Regulus took it with his bishop, but it took him much longer before he nodded slowly.

"It was Kreacher," he said eventually. Sirius nodded; Regulus had always had a soft spot for the house-elf, and both of them knew it. "I sent him off to help the Dark Lord, and the Dark Lord left Kreacher to die. But he came back." He paused, sighing.

"You ordered him to return?"

Regulus nodded. "He told me what had happened. The Dark Lord… he made Horcruxes, Sirius. Seven of them. I worked it out. And then I waited." He sighed, watching levelly as Sirius made his move, then brought his pawn forwards again to meet Sirius', his eyes fixed on the board. "That was the worst part. The waiting. I had to go on pretending I believed in what he was doing, had to go on pretending that I still thought we were right. It was… horrible. You don't know how horrible."

Sirius' mind went back to Azkaban, to the long days of waiting in that empty cell, and he shuddered. "I know, Reggie. Believe me, I know."

Looking up at his brother's face, Regulus nodded. "I suppose you do, don't you?" he admitted quietly. "But my chance came, and I took it. I made Kreacher take me to the island where the Dark Lord had left the locket – the Horcrux. I drank the poison he had put it in, and I left him a little present – a locket of my own, and a note of my own. And… I drowned." He laughed slightly, humourlessly. "It sounds stupid, when I say it like that. I drank poison, I destroyed a little piece of the Dark Lord's soul, and then I drowned. But that's what happened. The Inferi pulled me down, and I woke up here. Just like you woke up here." Again, that hollow little laugh. "You still think I don't know what hard to accept means, Sirius?"

Sirius looked levelly at his little brother, the gulf of years wide between them, and nodded slowly, sadly. "You're making this up. You're making this up, and maybe you believe it – no, not maybe – but you don't want to admit it. You don't want to admit that you fucked up, and…"

He was cut off as Regulus – Regulus who had always been the even-tempered, cautious one of them – leapt to his feet, slamming his hands down on the table so hard that the chess pieces wobbled and his king fell over.

"I'm not a liar, Sirius!" he shouted. "I wouldn't do that! I've done stupid things, and I've done evil things, but I have never! Ever! Lied! Not to myself, not to you, not to anyone!" His lip curling into a Black sneer that would have put Phineas Nigellus to shame, he leant on the table, breathing heavily. His dark hair, cropped short around the base of his skull, dangled across his face.

Sirius looked up at his brother, leaning on the table with angry spots of red on his high cheekbones and his slim-fingered hands clenching into fists, and saw altogether too much of himself there. That was Sirius, standing there. Young Sirius, to be sure, but Sirius nonetheless – Sirius after a fight with his family, defiant, upset, and absolutely in the right.

Reaching over, the older Black gently righted the black king, swallowing hard. "Regulus… Reggie. It's okay." He smiled, feeling tears rise slightly in his throat, and swallowed them back again. "I believe you. Really. I do. It's just…"

"I know." Regulus gave him a watery little smile, looking up. "I'm a Death Eater Slytherin bastard. Took me by surprise, as well." Sighing, he sat down heavily, his face in his hands. "I fucked up, all right, Sirius? I do know that. I fucked up, and I picked the wrong side. But the Hat was right in the end, wasn't it?" He bit his lip, looking up at Sirius and managing a much more genuine smile. "I changed things. I changed the world for the better." Again, his head sank into his hands. "I just wish I'd done it a different way. I should have chosen Gryffindor."

"Love and friendship, over changing the world for the better? Isn't that what you said the choice was?" Sirius smiled at his brother, looking more than a little watery-eyed himself. "Nobody with a conscience would have chosen Gryffindor out of those two, Reg. Nobody."

Regulus bit his lip. "But it didn't change anything, did it? The Dark Lord's still there. And I'm… well, I'm not."

Reaching over the table, Sirius put his hand on Regulus' shoulder, squeezing it comfortingly. "It doesn't matter, Reggie. You did the right thing."

"Dolce et decorum est, pro patria mori?" Regulus said bitterly, hanging his head. "I didn't change anything, Sirius. All I did was…"

"Was lay things open, so other people could get to him," Sirius interrupted, looking down at the chessboard. "Harry – that's James' son – he's going after the Dark Lord. And he'll kill him, I know he will. It's one less Horcrux he has to deal with, because of you." He smiled openly, squeezing Regulus' shoulder again. "You're supposed to be a chess master, aren't you, Reg? What did Uncle Cygnus always tell us was the first rule of chess?"

Regulus reached up to take his brother's hand, his smile wide and genuine now, although tears were still leaking from the corners of his eyes. "You can't play chess if all you've got is a king, or even a queen. The pawns are just as important."

Sirius nodded, smiling, and it could almost have been that Christmas, all those years ago. Letting go of Regulus' hand, he tapped the board, and smiled. "It's your move."