Regulus Black and the Horcrux Cave
Voldemort used Kreacher to test defences of the locket. Regulus didn't like this and wanted to destroy the horcrux. Only knew about the locket not the others. 1979 Regulus ordered Kreacher to take him to the cave and switch the horcrux with a fake one. Regulus drunk the poison and was dragged to his death by inferi. Kreacher switched the lockets and escaped. Kreacher couldn't destroy the horcrux so hid it in 12 Grimmauld Place.
Regulus was sitting in the drawing room of number 12 Grimmauld Place, reading a book about previous Ministers of Magic, waiting for his master to return with his house elf, Kreacher. Late in the afternoon a crack from downstairs. His slave had returned, Kreacher ran into the room and bowed to his master.
"Master Regulus, I have returned from my errand,"
"I can see that, tell me what you did,"
"I cannot Master, the Dark Lord does not wish me to speak of it,"
"He is not your Master, tell me what he had you do,"
"Yes, Master Regulus. He took me to a cave on the coast in some cliffs. We entered, he gave me a knife, I had to cut my hand and press it up against a wall. It revealed a lake-" The elf started before stopping to cough and he fell over.
"Get up elf and continue," Regulus barked at the old elf.
"My apologies Master Regulus, a massive lake was revealed and I had to pull a boat out of the lake and we traveled across the water to a little island." He stopped to cough again but quickly recovered and continued. "The Dark lord walked to a basin and ordered me to try scoop up the liquid but I couldn't, then he told me to drink its contents, a deadly poison," He coughed again but before he could continue his tale the black haired young adult cut in.
"A deadly poison? He tried to kill you? Why aren't you dead?"
"Kreacher was stopped after he started to go insane,"
"I wish he didn't, you old bag of bones. Did he have you do anything else?"
"Just row the boat back Master,"
"Did he say why there was poison?"
"To protect Salazar Slytherin's Locket, Master, it is crucial that no one destroys it,"
"Why?"
"Kreacher doesn't know Master, Kreacher must be punished," he exclaimed, hitting his head on the fireplace frame. Regulus ignored this and walked over to the old bookshelf, searching through the darkest books searching for a reason. Regulus had overheard his parents discussing something that splits the soul into something, he was sure it would be in the darkest book in this house. He picked up a book called 'Magick Moste Evile' but it had nothing related to immortality, though the introduction says something about horcruxes and that they are not being covered because they are to evil. Regulus searched every other book in the house, except for his fathers' private library which he didn't have the key to, but couldn't find anything. No immortality, no horcruxes, nothing.
"Kreacher!"
"Yes Master Regulus?"
"Unlock Fathers Library,"
"Yes Master, of cause," He answered before disappearing.
"That was easy, if I tried that last year it wouldn't have worked," he muttered, seconds later he heard a horrible screeching. "KREACHER PULL THOSE CURTAINS OVER MOTHERS PORTRAIT!" He screamed over his mothers' portrait, he hated the thing but it was stuck to the wall, literally, with a permanent sticking charm. He walked upstairs and entered his dead parents room, it wasn't very interesting, just a black and green bedroom with a bed, dresser, mirror, bedside tables and a wardrobe.
The thing he was interested in was behind the magically locked door that lead to the private library. A few minutes later Kreacher walked in holding the old silver key, he waddled to the door and unlocked it. The elf opened the door for his master.
"Took you long enough you useless thing," Regulus snapped, as he had been taught to when Kreacher was slow, and walked into the room. The room lit up as he entered. "Horcruxes, come on you have to be in here," he muttered, right after he muttered a book flew off a shelf and opened itself on a table. Regulus walked over to it and the page was exactly what he had asked for. 'Horcruxes, the darkest magic in this world. A horcrux is an item that a witch or wizard has hidden part of their soul in, making them immortal, unless the object was destroyed. In order to split the soul you need to…' Regulus stopped reading, not wanting to know how to rip the soul apart. He slammed it shut and stormed out of the room. "KREACHER WE ARE GOING TO THAT CAVE NOW!" He ordered, the thought of someone splitting their soul was so to much for him to handle. It was wrong, more wrong than his stupid older brothers morals. Regulus ran downstairs and snatched up his wand. "NOW!" He shouted and the house elf tumbled down the stairs to him, Regulus grabbed him and they disappeared with a crack. A few seconds later they landed on the shore outside of the horcrux cave.
"In there Master," Kreacher said, cowering as he looked at the cursed cave. Regulus took a deep breathe before entering, he shivered as he walked in, the air itself seemed to wreak of evil and darkness.
"Show me the way," he commanded, his voice breaking just as he finished speaking. The old elf walked over to a wall and picked up a rock, cutting his hand again and places it on the wall. The wall disappeared to reveal an endless cave of darkness.
"Lumos maxima," Regulus whispered, the tip of his wand lit up dimly, magic was dampened in here, it was impressive that Regulus could light his wand at all. "Get the boat," He instructed, his voice shaking, the cave itself caused fear beyond anything Regulus had ever felt, and he had fallen off a broomstick that was flying 1000 meters from the ground. He had survived because of Dumbledore, who, in a way, he was now helping by betraying his master, The Dark Lord, Voldemort. As his servant started to pull on a chain he grabbed it and helped. Soon a boat emerged from the lake, breaking the still water. They got in and sailed across the dark, endless lake, after what felt like an age they arrived upon a small island made of rock.
At the top was a marble basin, glimmering in the light from Regulus' wand. Regulus walked towards the thing of beauty while Kreacher refused to step off the boat. As Regulus reached the basin he called Kreacher and within seconds the elf was at his side.
"Is this the poison?" Regulus asked, not moving his eyes from the basin and the swirling green liquid within.
"Yes Master, it is the poison which Kreacher drank," He answered, grabbing a small shell cup and scooping up a mouthful of poison.
"You are not drinking that!" Regulus exclaimed, knocking it out of his hand, "I'd be damned without you," he added before grabbing the cup and drinking a mouthful of poison himself. He screamed as it slid down his throat, it felt like it was burning his insides with fire. "Kreacher, give me more," The wizard commanded, dropping the cup, Kreacher picked it up and scooped up some more of the potion, he took it to his master and forced it down his throat, Regulus fell to the ground screaming and wiggling from the pain. "MORE!" He screamed and he got more, this continued until there was very little poison left before he completely lost it, he started to crawl towards the water, despite his slaves warnings against touching the water. The dark haired mans hand lowered into the lake and it was grabbed by a rotting hand, he tried to free himself but the grip was to strong. "DESTROY THE LOCKET! DON'T LET IT LIVE!" Regulus Black was thrashing against the hand but it pulled him into the water where he was swamped by more rotting bodies, inferni, living dead. They drowned him before slowly turning him into an inferni like his killers. The elf was shocked at his masters disappearance and waited for him until the lakes surface was, once again, silent and still. His masters last words still echoing around the cave, "DESTROY THE LOCKET!" Kreacher scooped up the last of the potion before forcing it down his own throat, it burned him as well but there wasn't enough left to make him crazy. His long fingers scraped the bottom of the basin until they grasped themselves around a cold object, Salazar Slytherins' Locket, which held part of Voldemorts' soul. Kreacher escaped the cave without getting caught by inferni but despite his best efforts he could not destroy the cursed locket, and so the elf hid it away in 12 Grimmauld Place, what was the house of the Black family but now only housed the poor house elf.