The locket is described as a heavy, oval-shaped container made of gold. Its most distinctive feature is a serpentine 'S' mark on its cover, inlaid with small, glittering green stones. The locket was capable of being opened, though when shown to Tom Riddle by Hepzibah Smith, it was empty. After Lord Voldemort transformed it into a Horcrux, it became impossible to open through ordinary means or simple spells. It could only be opened by a speaker of Parseltongue.
As one of Lord Voldemort's Horcruxes, the locket contained a fragment of his soul, granting him a form of immortality. The portion of soul within endowed the locket with a malevolent sentience and several powerful defensive capabilities.
The locket was a priceless heirloom belonging to Salazar Slytherin, one of the four founders of Hogwarts. It was passed down through his direct descendants, the Gaunt family. The last of the Gaunts to own it was Merope Gaunt, Lord Voldemort's mother. In a state of desperation after being abandoned by her husband, Tom Riddle Sr., Merope sold the locket to Caractacus Burke at Borgin and Burkes for a mere ten Galleons. Hepzibah Smith, a wealthy but foolish collector, later purchased the locket from Burke. The young Tom Riddle, then working at Borgin and Burkes, visited Hepzibah and charmed her into showing him her two greatest treasures: the locket and Hufflepuff's Cup. Days later, Riddle murdered Hepzibah, stole both items, and magically altered the memory of her house-elf, Hokey, to take the blame. Riddle later used the murder of a Muggle tramp to turn the locket into his third Horcrux. Voldemort hid the locket in a secret seaside cave from his childhood, placing it within a basin of poison on an island in an underground lake, protected by numerous powerful enchantments, including a blood-toll entryway and an army of Inferi. However, his Death Eater, Regulus Black, learned of the Horcrux and turned against him. Accompanied by his house-elf Kreacher, Regulus overcame the cave's defenses. He drank the potion himself, ordering Kreacher to swap the real locket with a fake one and to destroy the original. Regulus was killed by the Inferi, and Kreacher, despite his best efforts, was unable to destroy the powerful object. The locket remained hidden at Number Twelve, Grimmauld Place until it was stolen by Mundungus Fletcher. To avoid prosecution for unlicensed peddling, Mundungus gave the locket as a bribe to Dolores Umbridge. She wore it at the Ministry of Magic, falsely claiming the 'S' stood for Selwyn, a pure-blood family to which she pretended to be related. After learning of its whereabouts, Harry Potter, Ron Weasley, and Hermione Granger infiltrated the Ministry of Magic and successfully stole the locket from Umbridge. They spent months carrying the cursed object until they discovered that the Sword of Gryffindor could destroy it. In the Forest of Dean, Ron used the sword to destroy the locket after Harry opened it by speaking Parseltongue.
The locket is a central plot device in the latter half of the series, representing Voldemort's pride in his Slytherin heritage and his fear of death. The quest to find and destroy it drives the narrative of Harry Potter and the Deathly Hallows, leading the main characters to locations like the Ministry of Magic and testing their friendship to its limits. Its destruction by Ron Weasley is a pivotal moment of character development for him, proving his courage and loyalty and marking a major victory in the war against Lord Voldemort. The fake locket, containing the note from R.A.B., serves as the key cliffhanger at the end of Harry Potter and the Half-Blood Prince, initiating the hunt for the real Horcruxes.