A/N: Prompted by "The One With the Boob Job (S9E16): Write about any member of the Black family."
Regulus wanted to scream. He wanted to kick the walls, he wanted to tear his hair out. He had torn some of his hair out, a stress habit he'd had since Hogwarts. He couldn't even blame exams or McGonagall this time; this was entirely his own fault.
Kreacher had come home in hysterics, shaking with tremors and unaware of who Regulus was, not even an hour after the Dark Lord had requested his use. Regulus had sobbed openly when he realized that, if he hadn't told Kreacher to "come back as soon as he's done with you," the elf would likely be dead. With Kreacher's help - describing the feeling of the locket, remarks the Dark Lord had made, not expecting the elf to be listening, or to live to share - Regulus had been able to decipher exactly what was hidden in that cave.
He had promptly run to the toilet and vomited.
Regulus had always known that the Dark Lord was… cruel. He seemed to delight in the torture of those who failed in service to him. He unabashedly wished to rid the world of practicing Muggle-borns. But to ask for Kreacher's help and leave him to die in some cave on the side of the ocean -
But that wasn't his worst offense. If Regulus was correct, and he was sure that he was, this locket was something even the books in the Black library danced around. Lord Voldemort had split his soul, and unless this locket was destroyed, he would be immortal. This Dark Lord would be impossible to defeat. Regulus wasn't sure when defeating - destroying - the Dark Lord had become a priority, but it certainly was.
He prayed his mother never found him out.
He had performed Legilimency on Kreacher, pulling the memory of the locket from his mind and drawing it from every angle, noting every scratch in its gleaming surface. He'd crafted a replacement and made small changes as he viewed and re-viewed the memories of the original.
"Kreacher?" The elf popped into his bedroom, hands still shaking from the after-effects of the potion. It had been over a week. "Does this look right to you?"
The elf nodded vigorously. "Yes, Master Regulus, this is looking quite like the one in the cave. Can Kreacher do anything else for Master Regulus?" he asked hopefully.
"No, Kreacher, I'm fine. Thank you," Regulus added. "I just wish that I'd been able to help that day, that I knew how to heal you. Hell, I wish I'd never gotten involved with this shit…" He put his head in his hands, speaking down to the table now. "I wish I could tell my younger self that it's not worth it. That the Dark Lord is a monster."
There was silence in the room for a moment. Regulus could dimly hear, downstairs, his mother ranting at his father. Something about Sirius, if he wasn't imagining things.
And then Kreacher was shifting his weight from one foot to the other, making the floorboards creak beneath him. Regulus jerked his head up to look properly at him. "Kreacher?"
"There are many things that elves can be doing," Kreacher said. His eyes were still downcast. "I might be able to be helping Master Regulus with such a task. Perhaps I could take Master Regulus back to talk to himself?"
Regulus's heart soared. This was the most hope he'd had in months - if he could stop himself from ever joining the bastard, Kreacher wouldn't get hurt. The Dark Lord would still hide the thrice-damned locket in the cave, and Regulus could tell his past self when and where to look for it! They could save so much pain, save so much time, and he would be at a safe enough distance to work against Lord Voldemort without immediate retribution.
"Try to travel back in time," Regulus encouraged. The elf looked pensive. Not a moment later, there were two Kreachers, each staring uncomfortably at the other. Regulus gaped. "Did you come back from the future?"
"Kreacher is from the future, yes, and Kreacher is also remembering being here," this new Kreacher said. He looked to his past self. "You should be leaving now," he told himself.
The original Kreacher snapped his fingers and disappeared. The Kreacher left behind was looking a little uncomfortable, but no worse for wear. It was a proof of concept.
It took several days of drafting, but finally Regulus had a letter that he felt would make clear to his younger self that he was going down a path he would regret. He transcribed, in excruciating detail, the people who had gone missing in the last three years, the treatment Kreacher had suffered at the Dark Lord's hands, the occasional torture he had undergone. The concept of Horcruxes, and what led him to believe that the Dark Lord had created one. His tentative plan to destroy it.
He held onto it for several more days, keeping it tucked in between the pages of an old Charms textbook in his childhood bedroom, charmed to look like a marked-up essay. At night, he would pore over it, gripping it tightly and imagining how things were going to change.
Finally, the day came. Regulus fought the urge to kiss the letter before calling Kreacher into his bedroom. He told Kreacher to answer any questions that young Regulus has, to help him help himself, to help him help Kreacher - to help him save the world.
Regulus gave the elf a well-earned hug and an almost brotherly pat on the back. With a snap of his fingers, Kreacher and the letter were gone.
"Kreacher?" The young Master Regulus's voice was hard. "What is this?"
Kreacher shifted his weight from one foot to the other. "Master Regulus has sent Kreacher from - from the future, you see, and -"
"Did Sirius set you up to this?" the young Regulus demanded. He slammed the letter onto his desk, his hand in a tight fist. Kreacher was beginning to have his doubts about this plan. "I order you to disregard whatever scheme Sirius has you playing a part in - didn't even realize he could still control you, he's been bloody disowned and he's still-"
"Master, this is not from your brother, this is-"
"Silence," Regulus ordered, and Kreacher couldn't help but obey. Regulus was furious, so angry that he was panting. He ripped the letter into two, and Kreacher shivered in his rags. "I'm sorry that Sirius has done this to you. I'm not angry at you, only at him. This is beyond inappropriate, and I will be speaking to him about this."
That's when Kreacher did what he felt was best. With a snap of his fingers, his dear young Master Regulus forgot the entire conversation. While he was still dazed, Kreacher vanished the letter off of his master's desk. It was only a moment before the young man before him fully regained awareness, and he looked at Kreacher oddly. "Is tea all the Master Regulus will be wanting?" Kreacher asked.
"Oh," Regulus said, brows together, clearly trying to fill in what had happened while he was apparently dozing off. "Er, yes, just tea is fine."
Tea appeared before the young Master Regulus, and Kreacher disappeared.
He reappeared in front of the only-slightly older Master Regulus of his own time. He bowed his head, deciding in that moment to lie to his master for the first time in his life. He hoped he would get away with it.
"I could not go back far enough, Master Regulus. I am so sorry." And Kreacher was sorry. Sorry that he couldn't make the young Master Regulus believe him. Sorry that he had burdened him with the knowledge of what had happened in that cave.
"It's alright, Kreacher," Master Regulus murmured. "I have a plan." Kreacher looked up, eyes brightened, until: "We'll be going back to the cave."
"Master, you-"
"Don't worry," Master Regulus said. "I know what I'm doing."