Helga Hufflepuff's Cup was a magical item created by one of the founders of Hogwarts School of Witchcraft and Wizardry, Helga Hufflepuff. The cup  has the dubious distinction of being the only object known to have been stolen from a vault (specifically the vault of the Lestrange family) at Gringotts Wizarding Bank.

Riddle, or Lord Voldemort as he called himself at that point, transformed the cup into his fourth Horcrux, using Smith's murder as the tool. Many years later he left it in the care of his trusted Death Eater, Bellatrix Lestrange.

Later, Harry, Hermione and Ron infiltrated Gringotts with the assistance of the goblin Griphook and the use of Polyjuice Potion. As they searched the vault, Harry felt a jolt in his heart when his wand light passed over Helga Hufflepuff's cup, which was kept on a higher level. They managed to narrowly bypass the vault's securities before Griphook betrayed the trio and claimed Godric Gryffindor's Sword, as well as alerting the other goblins to their presence. The trio was forced to fleeing Gringotts on one of the bank's dragons.

Destruction

The trio returned to Hogwarts Castle to find a possibly Ravenclaw-related Horcrux, and the Death Eaters and Voldemort launched an attack known as theBattle of Hogwarts. During the battle, Ron and Hermione ventured down into theChamber of Secrets (with Ron doing a passable imitation of Harry'sParseltongue to enter) and retrieved several Basilisk fangs. Hermione used one of the fangs to destroy the Cup inside the Chamber, as Basilisk venom is one of the properties capable of destroying a Horcrux.