A/N: Reg dies. We all know this. But I was like HEADCANON and needed to write it so... yeah.
FEELY FEELS AHEAD.
There were many things that Regulus regretted in his short eighteen years, becoming a Death Eater topped the list. Why had he become one in the first place? He could have stayed a supporter from the outside, he didn't need to go and get his mark.
How he wished that he could tell his seventeen-year-old self to turn around and never look back at Lord Voldemort. He wanted to go back to Hogwarts and stay there with the few friends that he had, marry a girl that he actually liked, and to be just a normal boy again. He wanted to go and burn all the Daily Prophet scraps he had about the Dark Lord so he could be done with him.
Mary. Mary MacDonald. Mary. Ever since his fourth year... He couldn't get the muggle-born out of his head. At one point, he had wanted to hate her. Oh, he wanted to kill her for being a muggle-born. He wished that she was a pure-blood, or even a half-blood so he didn't have to hate her. His mother might have approved of her even if she was a half-blood. They could have eloped if she didn't. No matter how much he wanted to hurt her... he could not do it. He could not bring himself to actually physically hurt her.
She was the only person that Regulus ever thought about fondly these days. He hadn't seen his cousin Andromeda in years, and she was the only one he could ever really deal with. She went off and married a muggle-born, that Ted Tonks. Another person he wished he could tell his fourteen-year-old self to be nicer to. Narcissa and Bellatrix were... well, Narcissa and Bellatrix. Both were insane in pure-blood frenzy, especially Bellatrix. She was at the Dark Lord's right hand. He adored her, and she quite obviously was more than fond of him.
"Kreacher, you know what we are here to to," Regulus said as he clutched the fake locket in his hand. "No matter what happens to me, you are not to stop giving me that liquid. You are to take the genuine locket and destroy it once I place the fake in the bowl. And when you apparate out of here to destroy it..." He took a breath, tears welling in his eyes. Regulus knew there was no way he would make it out of the cave. The Inferi would drag him in and he would die, there was no doubt about that. Even if he did manage to make it, he would die. Voldemort would make sure of that.
"When you apparate out of the cave with the horcrux, I want you to take this note to Mary MacDonald. You are not to tell my mother what I have done, you get that?" His hand shaking, Regulus handed the note to the House Elf. His only friend left telling the only girl he ever loved that he had defected and died trying to destroy the person who he had once admired over everything.
Dear Mary,
I know that I am the last person you would ever think you would hear from again, but you are the only person I want to know this. The house elf, Kreacher, is taking this to you because I am dead. I defected, Mary. I was wrong about Voldemort. I was so very wrong and I drove you away because I was a fucking git, believing in my parent's pure-blood mania and in what Voldemort was saying.
He is a horrid man, and it took me seeing my House Elf tortured to get that. I saw the man who I thought could do no wrong hurt one of the few beings that I ever really cared about... another being you.
I am dead, Mary, even if I get out of this cave. Voldemort will get me if I do. The locket that Kreacher has is called a horcrux. I don't know how many Voldemort has made, and I don't really think I want to know. But help him destroy it if you can.
I wish we could go back to when we were fourteen and you could kick the pure-mania out of me. Then maybe we could have died together. Good bye, pretty Mary. Stay out of the firewhiskey.
I love you.
Regulus Arcturus Black
"Master Regulus," the house elf said in his gravelly voice. "We are here."
Regulus got out of the boat and went straight to the center of the island. He immediately started to drink the liquid, the hellish liquid that would make him go insane and make him have a stronger thirst than strongest thirst he could ever fathom. "This is for all those that have died and will die after I do," Regulus said, voice quavering, as he was taking the first drink.
Many moments passed before he finished it and put the decoy locket in the basin. He held up the Horcrux for Kreacher. "Take it," he rasped. "Take it and destroy it. And take that note to Mary. And tell her... Tell her this was for all the times we shared."
He got up and desperately tried to take a drink from the lake, forgetting the Inferi in his hellish thirst. He screamed at the still house elf. "GO NOW! DON'T TELL MOTHER THAT I DIED FOR THIS." The Inferi dragged him to into the surrounding lake. "This was for Mary and all those who I wronged."