A/N: Not my sandbox, just my sand castle. I don't own anything from Harry Potter. Additionally, no one has permission to bind and sell my works, fandom or otherwise. Shame on you for exploiting other people's works!

I hope you guys enjoy this one-shot! TBH, I had a much more tragic ending in store for you guys (aka: Regulus dying despite Sirius' best efforts, but I decided against that.) There will be angst tho, so I hope you're prepared for that, muahahahah!

This started out at 3152 words, so BIG THANKS to my betas for helping me cut it down to under 3k. My muse always gets out of hand when it's on a roll lmao

Lots of love and thanks to Queenie, Sky, Hexibora, and Bea for the Beta!

Summary: When Sirius is given a second chance, his mind focuses on one of the brothers he really should have paid more attention to the first time around.

Triggers/Warnings: Inferi (rotting bodies/zombies), Angst/suspense (with happy ending)

Word Count: 2987

Prompts:

QLFC Beater Prompt:

Black tourmaline - The Color Black AND Write about protection.

Additional Prompts:

4. [action] cursing someone

6. [number] thirteen

8. [dialogue] "No." - "What?" - "You heard me. Get out." - "You'll die if I do."

Hogwarts School Of Witchcraft and Wizardry

[February Monthly] Play Your Cards Right

Black Jack - (color) black

[February Monthly] Single Bingo

(character) Regulus Black

[February] Writing Club

Superpower Hour

Alix Kubdel - Time-travel!AU

AU-wesome Adventures

[character] Sirius Black

Ongoing Challenge: Gather Your Party!

Wizard: Runecrafter: Plot Point: Learning a Language - alt: Relationship: Siblings

"The Last Thing I Do"

The aching black eye Sirius sported when he woke up only confused him more. He last remembered going through the Veil in the Department of Mysteries, Harry crying out for him reminding him of his late brother, Regulus.

Regulus. Only a year younger than him, Sirius had vowed to always protect him, and then had let house lines part them. Sirius knew it was because Regulus was pushed to be everything Sirius wasn't, but he couldn't help but resent his little brother for being a reminder of what could have been. Regulus was the physical reminder of the shackles that chained him to his family. As an adult, he had regretted every cold shoulder and cruel word, every prank that had turned his little brother's black Hogwarts robes to a garishly bright color. But back then? He was just a dumb kid who didn't think about consequences.

Kreacher's words to him before he left to go to the Ministry to save his godson rang through his ears.

"Master Regulus was a goodly master. He protected Kreacher, saved Kreacher. Kreacher failed Master Regulus! Kreacher couldn't destroy the Dark Lord's soul piece!" Sirius felt the color drain from his face and he knelt down to Kreacher's level.

"Kreacher, what happened to Regulus?"

"The Dark Lord killed him. Master Regulus ordered Kreacher to go home and to leave him. We got the soul piece, but Master Regulus was dragged under the water."

"Dragged under by what?" Sirius asked. Kreacher stared him in the eye, hatred burning for once not for Sirius.

"Inferi."

Sirius shuddered, feeling the intensity of Kreacher's gaze, even in his memories. He clutched at his blankets trying to ground himself.

"You alright there, mate?" came a familiar voice from his left. When Sirius looked, his heart clenched, seeing a teenage James Potter, messy black hair and all. James winced as Sirius turned to him. "Yeowch. Looks like Regulus did a number on you. What did you do to him again?"

Was this a dream? Was this the afterlife? Sirius searched his memories of when Regulus gave him a black eye. It must have been his last year, and Regulus' sixth year. It was the only time when Regulus looked stressed, and when Sirius had pushed, he had lashed out. Sirius raised a gentle hand to gingerly poke the bruised area.

"Guess I teased him one too many times," Sirius replied absently. The other boys began to stir, and Sirius started getting ready. If what Kreacher had told him was correct, Regulus would die next year. If he had really come back, this was his one and only chance to not only repair his relationship with his little brother, but also protect him. He could save Regulus from his untimely death. In the back of his mind, if he had actually come back in time–however unlikely that was…he could be dead–but in this scenario, why not do what he couldn't before. He could save James and Lily. He could help end this war. He could help protect everyone from that monster.

Sirius' expression hardened as he tied his tie. He just had to save his brother first.

Casting a Tempus charm, Sirius found out that today was May, Friday the thirteenth. With a slight groan, he grumbled. Of course today would be the unluckiest day. He had already gotten stuck in the trick stair on the way down to breakfast, and the boys had teased him mercilessly, claiming his head was in the clouds.

"What bird got you in such a bind?" James asked as Remus held open the door to the Great Hall for them. Peter trailed in after them as Sirius shrugged.

"None in particular. What if it's all of them?" he replied with a rakish grin. Was this how he was as a teenager? He was ashamed to do it now. His eyes looking over his classmates as they made it to their seats made his stomach roil. They all looked like children to him. They were children compared to him.

They sat and their day started like every other. James cuddled with Lily on the bench, Remus ate with a book propped up by a jug of juice, and Peter ate with gusto, but not impolitely.

Sirius' vision narrowed on their rat-like friend. Had he been a turncoat this year? Or was it only after graduation Peter decided to turn traitor? He shook his head to clear his thoughts. Now wasn't the time to figure out that problem. He looked for Regulus at the Slytherin table, spotting him near the doors to the Great Hall. Good, it looked like he was just starting breakfast.

Sirius quickly scarfed down toast and eggs before standing up.

"Where are you going?" Peter asked curiously. Sirius had to force himself to not snap back at his 'friend'.

"Gonna go meet a bird before class," he replied, the lie slipping from his tongue easily. James chuckled to himself.

"Have fun, Padfoot!" he called out before turning his attention back to his girlfriend.

With his bag slung over one shoulder, Sirius fought to not sprint out of the Great Hall. Pulling the Marauder's Map out of his bag, he watched his little brother's dot until it started to move. Sirius hid himself in an alcove next to the path he knew his brother would take, lying in wait.

When Regulus' dot was nearly on top of his own, Sirius reached a hand out from behind the tapestry and grabbed him, pulling him into the little alcove. Sirius yelped, letting go of Regulus as a sharp pain then numbness filled his arm. He stared at his wide-eyed little brother, who had his wand out.

"Are you here to get back at me for that black eye?" he asked.

"What curse was that?" Sirius asked curiously. Regulus stopped short as Sirius shook his hand out to get the feeling back.

"A lightning shock. It numbs the area, like Muggle anesthetic," Regulus mumbled. Sirius shot him a look.

"How do you know about that?" Sirius asked.

"Don't tell Mum!" Regulus exclaimed. Sirius snorted, and even Regulus laughed a bit at the thought of Sirius telling Walburga Black anything.

"I need to talk to you about something important," Sirius said, breaking the light mood they had started with. "I need you to stay away from You-Know-Who."

"No-"

"Following him is going to lead you down a path that you don't want to go," Sirius continued, not letting Regulus speak.

"It is expected-"

"He's going to get a whole lot worse. He started with political ambitions, but now he tortures followers for the slightest inconvenience! That's not a politician, that's a despot!"

"I bet Dumbledore-"

"Dumbledore locks people up if it's convenient for him!" Sirius exclaimed. They both fell silent as a sudden increase in noise of students leaving breakfast.

They both stared at the tapestry that covered their hiding place for a moment.

"What does it matter to you?" Regulus demanded, voice hushed.

"You-Know-Who made soul anchors. It's why he's confident he'll never die," Sirius said quietly. Regulus blinked at him in horror.

"How do you know about them?" he asked, his voice slightly strangled.

"That's why you have to get out. Stay away from him. I know you won't get your Mark until this summer. Come to the Potters with me, you don't have to–"

Regulus glared at Sirius, taking the older man aback. "No."

"What?"

"You heard me. Get out."

"You'll die if I do." Sirius' voice was quiet as he stared at his baby brother, pleading with his eyes. The sincerity of his voice stopped Regulus short.

"What do you know?" he asked warily. Sirius sighed in relief. For a moment there, he feared Regulus would have walked away without fully hearing him out.

"I'm from the future. Bellatrix killed me, sent me through the Veil of Death–"

"No one's survived that," Regulus pointed out.

"I did," Sirius replied. "In my future, you disappear this coming summer. You retrieve one of…his soul anchors. Kreacher is unable to destroy it, and you die in vain. Please, little brother. Let me help you. I've done a shoddy job of protecting you at Hogwarts. But now I know this is my second chance. Let me do what big brothers should do and keep you safe."

Regulus thought a moment before taking a deep breath and exhaling. "Okay. You can help me. But you need to tell me everything."

They began to meet in secret, and then more overtly as the school year wore on. Surprisingly enough, no one cared that the brothers spent more time together. The blood purists thought Regulus finally wore Sirius down to come to their side, and Sirius' friends thought Sirius got through to his brother to join him in his rebellion against their parents' restrictions.

Neither were correct, but it was somewhat of a middleground. Regulus had become disenchanted by their mother's vitriolic rants several years ago, but his fear of her wrath was greater than his desire to get out from under her thumb. This Sirius was more grounded than the brother Regulus had punched for pulling a cruel prank on him.

"How many do you have there?" Sirius murmured as he counted out the list of possible soul anchor locations. Drat, he had thought up thirteen of them. Sirius scratched out one of the less likely places, to make an even twelve.

"Thirteen," Regulus replied, recounting his own list of possible items He-Who-Must-Not-Be-Named might have used for his soul anchors.

Sirius swore aloud, ignoring Regulus' warning stinging hex. He just couldn't escape the number thirteen. From now until Harry's first year of Hogwarts–and You-Know-Who's first attempt at gaining a body–was also thirteen years. He just couldn't escape the number since coming back. It was an unlucky number, and Sirius hoped that their mission would go well in spite of it. Regulus threw his parchment and quill onto the desktop in frustration, startling Sirius out of his thoughts. Running his hands through his hair, Regulus blew out a frustrated breath.

"I think we need to ask for help," he stated.

Sirius grimaced. "If we had access to the Black library–"

"Which I only will if I go back this summer, and you know how hard it is to escape once Mother has her claws in you," Regulus replied. "And it's for that same reason that you can't go back either. She'll lock you in the basement until you come to heel."

"If only we could get one of the Potter elves in there–" Sirius started, but Regulus' grin made him pause.

"I can do one better," he said. "Kreacher!" he called out into the empty classroom they had been working in. A slight pop heralded Kreacher's arrival.

"Yes Master Regulus?" asked the house-elf.

"Kreacher, I need you to do something for me."

He stared at the fading patronus momentarily as his brain tried to comprehend the words. It's go time. A second later, Sirius sprang to his feet and grabbed his black dragonhide vest. Donning it under his shirt, Sirius checked himself to make sure he wasn't forgetting anything. He and Regulus had planned to meet at the park across from Grimmauld Place as soon as Kreacher came back to him from his excursion in the cave. They would go together and begin their plans of foiling the Dark Lord. Apparating away, Sirius hoped everything would be alright.

"Alright, you know the plan. I drink the potion, you switch out the locket, and Kreacher gets you out of here. He'll take you back to my flat. I've heard Fiendfyre can destroy it, or Basilisk venom, but that's hard to get that, so-"

"No."

Sirius scowled at Regulus' glare. "What?"

"You heard me. We're sticking with the plan," Regulus said. "I drink the potion. You switch the locket. You get out."

"You'll die if I do," Sirius said, echoing his earlier words from months ago.

"Masters must work as a team," Kreacher interrupted, glaring at both of them. "Together you will succeed. Separate, Masters will fail." Both men huffed but agreed with the house-elf. Regulus would drink the potion, Sirius would switch the lockets, Kreacher would get them both out at the same time. Nothing could go wrong.

But Sirius should have remembered that today was Friday the thirteenth.

Sirius paled as he saw Regulus crawl towards the water. "No!" he screamed out. He thrust the locket into Kreacher's hands and exclaimed, "Kreacher, I order you to take this and Regulus and get out of here! Get him somewhere safe!" Without a word, Kreacher did as he was bid, popping to Regulus in a heartbeat, mere centimeters from his hand touching the water. Regulus had a moment to stare at Sirius, betrayal on his face at the resolution on his brother's, and then they were gone.

The silence in the cavern aside from Sirius' heavy breathing felt oppressive. The tension in Sirius drained out as the enormity of the situation hit him. Kreacher wouldn't willingly come back for him. He hadn't ordered the house-elf to come back for him, and Kreacher only tolerated him for Regulus' sake.

He was going to die here.

Sirius took a moment to calm himself down, his heart still hammering from imagining his little brother being taken by the Inferi once more, despite his best-laid plans. But in the end, he did it. They got the Horcrux. He saved Regulus. He still had to protect James and Lily and baby Harry from Peter's betrayal, but he had left Regulus detailed notes about that. They would survive, even if he didn't. Now, he just had to find a way out of here: and if he couldn't, collapse as much of this cavern as he could.

As if sensing his thoughts, a pulse of magic suddenly surged through the cavern from deeper in, a wave of dread filling Sirius. The surface of the water rippled, little drops every few feet, and then larger ripples. Suddenly a hand emerged from under the water, reaching for him.

Bracing himself, Sirius readied his wand with the first syllable of a fire spell on his lips. Fire was their weakness, which was probably why Voldemort submerged them in water. The only thing hot enough to destroy waterlogged Inferi would probably be Fiendfyre, but Sirius knew he couldn't control it. He'd summoned it before, but he would never be able to control it.

He'd tried.

However, he was prepared to die here. Sirius had backup plans in place, and would sacrifice himself if needed.

When the first fire spell merely made them screech instead of catch fire, Sirius felt true fear, for the first time since coming back. It wasn't a matter of if, but when they would get him, and he was determined to not go down without a fight.

His wand slashed through the air in broad strokes, piercing spells and curses spilling from his lips as they surrounded him. "Sectumsempra!" he intoned, remembering the curse that Snape had missed him with the week prior. It shredded three of the Inferi and a flame of hope rose in his chest. He threw out a few more, cutting down more of the rotting bodies ambling towards him, but the spell took a lot of his energy and he couldn't keep it up forever. Doing enough to get him some breathing room, Sirius took a deep breath and closed his eyes.

This next spell would be all or nothing. His energy was flagging. But he had one last curse he could use that could incinerate a good chunk of the Dark Lord's horde. Even just thinking the incantation made him feel slimy, but it was a spell he found in the library of Grimmauld Place when they had been cleaning it up, back in the other timeline. Why his mother had a record of it, he didn't know, but he was grateful for it now.

"Protego Diabolica," Sirius intoned, twirling his wand around in a circular motion. Black flames spewed forth and hit the ground where he directed it, creating a perfect circle around himself. Sirius panted from the effort and slumped where he stood. The first screech of an Inferius as it tried to cross his circle of fire brought a smile to his face. Opening his eyes, he watched as rotting body after body threw themselves onto his circle of black fire to try to reach them. Even waterlogged, they were no match for this dark charm.

With a gasp, Sirius' energy failed him at the wrong moment, falling to his knees to catch his breath. The black flames died out, leaving behind a darkened cavern and several Inferi still dragging themselves out of the water. Thirteen of them, he noted idly, his body heavy.

They drew ever closer, and his heart quickened in fear. Squeezing his eyes shut tightly, Sirius saw the faces of all of his loved ones pass before him for the last time: Regulus, Remus, James, Lily, Harry…it was for them that he was here, and for them that he would willingly die a thousand times over so long as it meant they were alive and safe.

Sirius felt a hand grab the back of his shirt, nails scraping at his protective dragonhide vest, and prepared himself for the worst. But the unmistakable feeling of elven apparition pulled him to his feet. His eyes jolted open. The sitting room of his flat stared back at him, and it took him a moment longer to realize that he was home and out of the cave.

"I should curse you right now," Regulus groaned from behind him. Sirius whirled around to see Regulus lying on the couch, guzzling a glass of water.

Sirius grinned at him, a laugh bubbling up at the realization that they had succeeded.

"One down, five to go. Love you too, little brother."