This medallion is a Black family's heirloom. With the help of his house-elf Kreacher, Regulus exchanged it with Salazar Slytherin's Medallion, which was hidden in the sea cave, in an attempt of destroying Voldemort's Horcrux and breaking down the Dark Lord. Regulus was killed in the process when he was dragged underwater from the Underworld, but Kreacher was able to successfully swap the medallions and escape. Regulus' medallion was found by Harry Potter and Albus Dumbledore in June 1997 when they also attempted to obtain the Horcrux. Harry gave it to Kreacher the following August, as an act of kindness.