Item from harry-potter-half-blood-prince by
A Horcrux — or so Dumbledore and Harry thought. The one they retrieved from the cave is a fake. The real one was taken by R.A.B.
Dumbledore and Harry endure terrible dangers in a seaside cave to retrieve the locket — Dumbledore drinks a basin of agonizing potion, they fight off Inferi. But the locket is a fake. Inside is a note: 'To the Dark Lord: I know I will be dead long before you read this but I want you to know that it was I who discovered your secret... R.A.B.' — Regulus Arcturus Black.
A heavy gold locket with an ornate serpentine S on the front, in green stones.
Also known as: the locket, Slytherin's locket