First skirmish at the Cave

The first skirmish at the Horcrux Cave occurred in 1979, when the disillusioned Death Eater, Regulus Black, and his house elf, Kreacher, aimed to retrieve and destroy Salazar Slytherin's Locket, which Lord Voldemort had placed in the cave. While the locket was successfully removed from the cave by Kreacher, Regulus lost his life in the attempt, and the locket was not destroyed for another eighteen years.

Background information
As a child, Tom Riddle was brought to the sea side for an outing, along with several other children from Wool's Orphanage. Riddle soon discovered a cave among the cliffs, where he took two other children, Dennis Bishop and Amy Benson, terrorising them to the point that they were unable to speak of what happened. It is likely that he only managed to reach the cave with magical means, scaring the other children in the process of taking them with him.

By 1979, Tom Riddle had grown up to become Lord Voldemort, and the First Wizarding War had been raging for nearly a decade. In the contravening years, Lord Voldemort had created five Horcruxes in an attempt to attain immortality, and Regulus Black, who had recently graduated from Hogwarts School, had joined the Death Eaters. In the course of securing a hiding place for one of his Horcruxes, Voldemort asked his Death Eaters for a House-elf. Regulus volunteered Kreacher, the House of Black's aged, loyal elf, telling him that it was a great honour to assist the Dark Lord. Kreacher was brought before the Dark Lord, and the two ventured to the cave by the sea. In order to test his carefully constructed defences, Voldemort used Kreacher's blood to open the cave and made Kreacher drink the potion from the basin on the island. Kreacher was consumed by terrible pain and relived horrible memories as he drank the potion. With Salazar Slytherin's Locket in place, Lord Voldemort left Kreacher on the island either to die of thirst or at the hands of the Inferi in the lake. However, Kreacher was able to apparate back to 12 Grimmauld Place once his mission was fulfilled, as Regulus had ordered him to, since the magic that bound Kreacher to obey his master superseded all magical protections placed on the cave to prevent typical apparition in or out. House-elf magic was something Lord Voldemort considered far below his notice; therefore, he had not anticipated Kreacher surviving.

Regulus had been able to deduce the existence of Voldemort's Horcruxes from the vague references Voldemort made to his immortality. After hearing Kreacher's tale, Regulus decided to seek vengeance against the Dark Lord by setting out to destroy his Horcruxes, beginning with the one for which Voldemort had caused Kreacher such great pain. He ordered Kreacher to lead him back to Voldemort's cave by the sea.

The Skirmish
Regulus and Kreacher set out to the Horcrux cave, Kreacher for the second time. As Regulus had ascertained that Lord Voldemort was keeping a locket in the cave, he brought with him a copy of the Horcrux, which he ordered Kreacher to switch with the original after the potion had been drunk. Regulus also told Kreacher to leave the cave and destroy the Horcrux, without telling anyone within the Black family what had transpired. Kreacher did as he was told, but not before witnessing Regulus dragged into the water and killed by Inferi after consuming the Drink of Despair.

Aftermath
Kreacher returned to 12 Grimmauld Place to fulfill his orders. However, he could not destroy the locket as he was ordered to do and punished himself severely for it. He realised that he must open the locket in order to destroy it, but as no magic he used could accomplish this task, he eventually hid the locket within the Black home.

His master's death was something that Kreacher had to live with for the rest of his life, and he was reduced to sobs when he spoke of it. The Black family searched for Regulus to no avail, and Kreacher could not inform the family of the happenings in the Horcrux cave as Regulus had ordered him to keep his silence. The loss of Regulus was incredibly hard on the Black family.

By 1996, Albus Dumbledore had also guessed the Dark Lord's secret and scoured the country for the Horcruxes. That summer, he was cursed while obtaining Marvolo Gaunt's Ring; knowing that death was imminent, Dumbledore taught Harry Potter about the Horcruxes and what they might be. In June 1997, they went to the cave by the sea to retrieve a Horcrux, not knowing that it had been replaced with a copy. Dumbledore was killed that night by Severus Snape; only after his death did Harry Potter discover that the Horcrux was a fake and that the real one had been stolen by the mysterious "R.A.B."

In the summer of 1995, the Order of the Phoenix took the Black family home at 12 Grimmauld Place as their headquarters. In their efforts to make the residence habitable, the Order, under the direction of Molly Weasley and Sirius Black, waged war on the house, throwing away many heirlooms, including the locket. The locket was saved by Kreacher, but when Sirius died, Mundungus Fletcher took possession of the locket after ransacking the House of Black for any valuables. He was selling the items on the street when Dolores Umbridge appeared and asked him if he had a license to do so. She let him off once he bribed her with the locket.

After they had discovered R.A.B.'s identity, heard Kreacher's tale and tracked down Fletcher, Harry, Ron and Hermione infiltrated the Ministry upon realising that Umbridge had the locket. They stunned Umbridge, replaced the locket with a decoy, and stole the real Horcrux before going on the run.

Later, the trio realised that one of the ways to destroy Horcruxes was with Godric Gryffindor's Sword, as it had been impregnated with Basilisk venom after Harry slew the Basilisk in the Chamber of Secrets. Dumbledore left the sword to Harry in his will, but knowing that the Ministry would not give it to him, entrusted Professor Snape with the task of getting the sword to Harry. In the winter of 1997, Snape discovered the Harry and Hermione were camping out in the Forest of Dean; he placed the sword not far from their tent and led Harry to it with his doe Patronus. The locket attempted to kill Harry before he could obtain the sword, but he was saved by Ron at the last moment. At Harry's insistence, Ron destroyed the locket shortly after Christmas of that year.

Appearances

 * Harry Potter and the Deathly Hallows