Disclaimer: The characters, places, etc. were created by J.K. Rowling.

A/N: A big thanks to PigWithHair, who beta'd this story for Checkmated.

Blood Brothers

Sirius hurried to the peephole. The pounding on his door was as frantic and persistent as the rain that had been assaulting London since early that morning. Distorted by the glass he saw a figure huddled by the door, desperate for sanctuary. It was a figure Sirius was sure he seen before, but he knew it was impossible for that person to be here now. Wand gripped firmly in his hand, he carefully opened the door, allowing the drenched refugee to rush in.

It took Sirius only a moment to recover from his initial shock, and he hastily shut and locked the door behind him.

"Are you crazy? What the hell do you think you're doing here? Do you want to get us both killed?" he hissed.

The resemblance between Sirius and Regulus Black was astounding. They shared the same aristocratic features, pronounced cheekbones and dark hair. The main physical difference between the brothers, besides Sirius' slight height advantage, was their eyes. Sirius' stormy gray ones now stared angrily into the brilliant blue of his brother's.

Regulus, it seemed, was in shock. His eyes were wide and unfocused, fearful even. Sirius couldn't remember ever seeing him like this, and it unnerved him. "What are you doing here, Regulus?" This time, he grabbed his shoulders and jerked him back, trying to remove him from his trance.

He finally seemed to focus on Sirius, though his eyes remained wide. "I was wrong," he whispered. "I was so wrong."

"What are you talking about? Why the hell did you come here?" Despite the way he often acted, Sirius cared for his brother. It was this oft-buried affection that forced his harsh tone: if Regulus, a Death Eater, was found in the home of an Order member, they would both surely perish.

Regulus seemed to finally snap out of his stupor. "I'm sorry, I- I didn't know where to go." He was shaking, but it was clearly not from cold.

--

Regulus was just shy of ten years old when Aunt Druella, acting on information from his cousin Narcissa, sent the letter about his brother.

Sirius, it seemed, was a Gryffindor.

His mother screamed for days about the latest development in the life of her rebellious eldest son. Any howlers Sirius received at school couldn't convey half of her anger and disappointment, which meant Regulus heard the most of it. It didn't really bother the boy: his mother had been yelling on a daily basis for as long as Regulus could remember, so he'd grown quite used to the sound. And besides, he wasn't the one who'd been Sorted in the wrong house and befriended blood-traitors, so he knew he had nothing to worry about.

It was his father's reaction that scared him. Orion Black had always been a quiet, stoic man, never one to show affection or much emotion at all. When he heard about Sirius, he remained stony-faced, as always, allowing his wife to rant while clearly not listening to a word of it. However, there was something about his eyes that terrified Regulus.

Later that night, Regulus had wandered downstairs to get a glass of water. As he tiptoed through the quiet corridor, he perceived a light from the sitting room. There, in the same position he'd been in hours ago, was Orion, ice clicking together in the otherwise empty glass he held. When the man turned and saw his son lurking in the doorway, he stood. His cheeks were pink, a strange color to see on such a formidable man. At least, that was the last thing Regulus remembered think before a strong hand came down across his cheek.

Too shocked to move, Regulus merely stared up at his father, petrified. The older man, on the other hand, seemed to have been jarred by this and looked up as if realizing where he was for the first time in days. He briskly strode out of the room, and Regulus returned to his bedroom. He tried to stay out of his parents' ways for the next week, sulking in his bedroom until his bruise healed.

--

"You couldn't hide out with one of your Death Eater buddies?" Sirius asked harshly, immediately wishing he could take it back. His callousness was a long-nurtured attribute, a defense mechanism against his family, but he hadn't meant to use it just then. His brother barely seemed to notice as his eyes darted fearfully around the room.

His brow furrowed, Sirius studied the boy's face. Honestly, that's all he was: a boy. He'd become a Death Eater when he was only sixteen, a choice Sirius had always felt he must have been pressured into. Yes, Regulus had believed his parents' prejudices and thought himself better than Muggle-borns and half-bloods. But he'd never been malicious or violent about it, and Sirius secretly hoped that hadn't changed.

Regulus himself had realized within a few short months that he was in over his head. Due to the loyalty of his cousin Bellatrix, Regulus had been immediately trusted and brought deep into Voldemort's inner circle, where he learned the twisted truth behind the organization he'd become involved in. The Dark Lord despised Muggles, mudbloods, half-bloods, and blood-traitors, like most of the Black family. However, it seemed that it was not a superiority complex like it was for his parents; Voldemort had something deeply ingrained in his psyche, a need to destroy all these beings and prove himself as their better that Regulus couldn't, and in fact didn't want to, fully understand.

He leaned in close and his voice lowered to a whisper, as if afraid that he'd be overheard. "I've left."

--

"Sirius!" The name seemed to bounce off the high stone walls in the corridor, which was nearly deserted save for the suits of armor stationed along one side and the two dark haired boys.

Sirius turned around sharply to face his brother, a fourth year Slytherin. Though it was only a few days into the new school year, they hadn't spoken in well over a month. "What do you want?" he asked, his voice lofty and bored.

"I- well, I wanted to see how you were," Regulus replied, stumbling over the words.

"Fantastic now that I live with the Potters," he said with mild condescension.

Regulus remembered that day perfectly. Despite the raging storm outside, Sirius had, after a nasty row with their mother, gathered up his things and marched out the front door defiantly. In a fit of rage, Walburga blasted his name from the family tree, and the only members of the Black family who'd heard from him since were those who had been similarly excommunicated. "You could still come back, you know. They'd let you come home."

"I don't want to go back. And I don't know why you stay. That place isn't a home, it's a prison. What've they ever done for you anyway? For either of us?"

Regulus looked down. "They're our parents, Sirius."

Sirius' face was stiff with suppressed anger, and for a moment he reminded Regulus of their father. "No. They're just people who lent us some genetic material, let us live in their house. They were never really parents."

With that the older boy turned abruptly, leaving his poor brother to wonder if he realized what those rash actions had cost them both.

--

Sirius stood for a moment, shocked. "What do you mean you've left?"

"Voldemort. The Death Eaters, I've left them," he said with a deranged, strangled sort of laugh.

"Left, you- you don't just leave Voldemort!" Sirius cried. "He won't let you, he'll kill you, Reg!"

"Yes. Yes, he'll kill me." He once again looked frightened, as if the full weight of this fact had settled on him. "He won't let me go, I know too much. But there's something I have to do first."

"What are you on about? Have you lost your bloody mind?"

Regulus, however, was not listening. He was growing anxious, bouncing on the balls of his feet like he had when he was younger. It was a nervous habit that had annoyed Sirius to no end. "He wants to be immortal, indestructible. He's been working on it for years. You have to tell your friends that."

"My friends?"

"They need to know. I don't know exactly what he's doing, but it's dangerous. Even more so if he succeeds." He stopped to take a breath, and once more surveyed the room as if looking for attackers to come at him at any moment. "There's a spy, too, I'm sure of it. I don't know who, but—"

"Regulus!" Sirius' commanding tone silenced him, but did nothing to calm his nerves. There were a million different questions Sirius wanted to ask him, all seemingly more urgent than the last. However, the one that came out was, "Why are you telling me this?"

Regulus looked at him strangely, as if the answer were the most obvious thing in the world. "Because we have to stop him."

--

The sharp intake of breath sounded like hissing, perhaps unsurprising from a Slytherin. "Sirius!" she had cried, gently placing her hand on his arm. Regulus hissed in pain, recoiled from her touch, and spun around sharply to face the startled green eyes of the Head Girl.

"Sorry, I thought you were—"

"I know who you thought I was, Mudblood!" he said, cross with her for the pain she had unknowingly caused more than anything else. The others said the burning would go away in a few days; unless, of course, the Dark Lord needed them.

Lily Evans had, in fact, been searching for the boy's older brother, who Regulus was mistaken for often enough to irk him. This fact only added to his displeasure at both his brother and the Head Girl

Sirius, however, showed up at just that moment, and his timing probably couldn't have been worse. "Hey! Get your bloody hands off her!" Regulus, who hadn't even realized that he'd grabbed her wrist angrily, now roughly released her. She stepped back as Sirius came face to face with Regulus. For a moment, they simply glared at one another, as if they were enemies instead of relations.

Lily was shocked by the obvious animosity between the two. Despite her own sibling problems, she'd never looked at Petunia the way Sirius did Regulus at that moment. "Five points from Slytherin for language," she said calmly, and tugged hard on Sirius' arm, forcing him to walk away with her.

As they left, Regulus watched her link her arm amiably with his brother's, and began talking quickly to try and calm the young man. It clearly worked, as Sirius' barking laughter soon filled the hall. He narrowed his eyes at the pair slightly, a strange jealousy erupting in him, for the way they were speaking, it was almost as if that Evans girl thought Sirius was her brother and not his.

At this point, however, that might not have been too far from the truth.

--

"I was wrong, and I know that now," he said, and his confessional tone stopped Sirius from interrupting. "But I'm being like you, Sirius. I'm being brave. For once I'm doing the right thing."

"But at what cost?" he said angrily. Strange, he thought briefly, that he should try to talk his brother out of leaving the people he fought against daily, the mass murderers he despised.

"What does it cost you?" Regulus replied sharply. His blue eyes looked worn, aged beyond what they should have been. He was too young; they were all too young for this life, this war. And they all knew the risks involved. "Sirius, I… I'm sorry."

It was unclear what he was apologizing for, but they didn't have much time to ponder the details now. "Where are you gonna go?" Sirius asked quickly.

"I don't know. I have to finish something, then… I haven't thought of what else I can do yet."

"You need to get as far away from here as possible," Sirius said urgently.

"I doubt I'll have the time for that. They'll figure it out, they'll find me." He once again looked around the room uneasily. "You should lay low, Sirius. They know about you, know you would never join them. Bellatrix, especially, would love to do you in, so you have to be careful."

"You too."

Regulus placed his hands on his brother's shoulders, looking him squarely in the eye. Steadily, as if most sure of himself at that moment, he said quietly, "You were the good one, Sirius."

Sirius watched, unable to speak as Regulus rushed out the door. As quickly as he'd appeared, he was gone, disappearing into the black night, and somehow Sirius knew with a sickening certainty that he would never see his younger brother again.