The locket was a precious family heirloom for the Gaunt family, who were descendants of Salazar Slytherin, and was worn by Merope Gaunt. Merope sold the locket when she arrived, pregnant and homeless, in London to Caractacus Burke of Borgin and Burkes in Diagon Alley.
The locket was in the possession of Hepzibah Smith, who purchased it from Borgin and Burkes for an extortionate amount. It was shown to Tom Riddle during a visit to Hepzibah. The locket was found to be missing after Hepzibah was murdered. Professor Dumbledore suspects that Tom Riddle stole the locket, along with Hufflepuff's golden cup.
Dolores Umbridge wears Slytherin's locket around her neck at the trial. She says that the "S" stands for Selwyn, one of many pure-blood families to whom she claims to be related. It is taken from the unconscious Umbridge by Hermione as she and Harry escape the courtroom.
When Harry dives into the forest pool with the Horcrux locket still around his neck, it constricts and begins to strangle him. It is cut off Harry by Ron after he pulls him out of the water.
The locket must be opened using Parseltongue. Behind both the glass windows of the locket, a living eye stares out. The presence in the locket speaks to the person attacking it, voicing their deepest, darkest fears.
The Horcrux is destroyed by Ron Weasley with the sword of Gryffindor. There is a clang of metal and a long, drawn-out scream when the locket is shattered by the sword.