Horcrux locket

Voldemort used the Black family house-elf Kreacher to test the locket's defenses. When he had seen the cave's effectiveness in action, he left Kreacher to die on the island in the middle of the dark lake. However, he forgot that house-elves have special magic binding them to their masters, and Kreacher was able to Apparate to his master Regulus Arcturus Black, one of Voldemort's Death Eaters. Black was infuriated at the mistreatment of Kreacher, and turned against Voldemort. He forced Kreacher to take him back to the cave where he crossed the lake with the house-elf and drank the potion himself. He instructed Kreacher to switch the true locket with a fake one, and to take it home and destroy it. He ordered Kreacher to sail across the lake without him, and Kreacher, with the Horcrux in hand, watched in horror as Black was dragged beneath the lake by the Inferi. Kreacher was able to escape to the Black family home, 12 Grimmauld Place.

Kreacher was unable to destroy the locket, thus he placed it in a glass case in the drawing room and left it undisturbed for many years until the Order of the Phoenix began to use the house as headquarters. The Order undertook a massive, overdue cleaning of the house, and the locket was placed into a rubbish pile to be thrown away. Kreacher secured the locket in the cupboard where he slept, along with some other Black family heirlooms that were headed for the trash.

After the death of Sirius Black in 1996, Mundungus Fletcher combed the house and stripped it of its remaining valuables. These included the locket from Kreacher's cupboard. He took it to Diagon Alley and began to sell his wares, and was caught by Dolores Umbridge. Umbridge eyed the locket and thought she could pass it off as a Selwyn family heirloom, as evidence of her alleged pure-blood status. She agreed to let Fletcher off for the obvious thievery in exchange for the locket.