“He looked at the serpentine S, inlaid with glittering, green stones: It was easy to visualize it as a minuscule snake […] concentrating on the letter S, imagining a serpent, while the contents of the locket rattled like a trapped cockroach. It would have been easy to pity it, except that the cut around Harry’s necks still burned […] The golden doors of the locket swung wide with a little click. Behind both of the glass windows within blinked a living eye, dark and handsome as Tom Riddle’s eyes had been before he turned them scarlet and slit-pupiled.”
—Description of the locket before it is destroyed by Harry Potter and Ron Weasley.
Slytherin’s Locket was a piece of jewellery originally owned by Salazar Slytherin that became an heirloom of his family. It was a locket of heavy gold with a serpentine S in glittering green stone inlay on the front. After descending down from Slytherin, it came into the possession of the Gaunt family, and Marvolo Gaunt treasured this heirloom of this pure-blood heritage along with his ring heirloom. After Marvolo and his son Morfin Gaunt were sent to Azkaban, the locket was stolen by Marvolo’s daughter Merope Gaunt, who sold it to Caractacus Burke for ten Galleons, a price far below what the priceless artefact was truly worth.
The locket was bought from Burke’s shop, Borgin and Burkes, by a wealthy witch with an affinity for rare or valuable items named Hepzibah Smith. In the mid-Twentieth century, Smith showed it to a young Tom Riddle, an employee of Borgin and Burkes whose job it was to get people to sell their treasures to the shop, though Hepzibah was firm that she was not selling the locket. Riddle was clearly interested in the locket (which was, after all, his birthright), and two days later, Hepzibah was found dead and her house-elf, Hokey, made a full confession (Riddle presumably performed a False Memory Charm on her, as he did with his uncle Morfin years earlier). Using the murder of a Muggle tramp, Riddle turned the locket into his third Horcrux and hid it in a seaside cave. It was taken from the cave, however, by a defected Death Eater named Regulus Black with the attempt of having his house-elf, Kreacher, destroy it, but the elf was unable to.
It was stolen from 12 Grimmauld Place, the home of the House of Black where Kreacher had kept it, by Mundungus Fletcher, who pillaged the house for valuable heirlooms after the last member of the Black family, Sirius, died. During Harry Potter, Ronald Weasley, and Hermione Granger’s attempts to find and destroy Voldemort’s Horcruxes, they decided to track down the locket first after retrieving the fake one placed by Regulus from the cave. They learned from Mundungus that it was taken from him by a Ministry bureaucrat named Dolores Umbridge, and thus the trio infiltrated the Ministry, found Umbridge, and retrieved the locket. Ron retrieved Godric Gryffindor’s Sword from the bottom of a frozen pool (after Harry was almost killed by the Horcrux while attempting to bring the sword up) and was able to destroy the Horcrux with it (the sword was imbued with Basilisk venom, one of the few substances capable of destroying Horcruxes) in the winter of 1997.