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Golden Horcrux hidden in Bellatrix's vault at Gringotts -- protected by a dragon, Gemino and Flagrante curses, and the entire goblin banking system. Destroyed by Hermione with a Basilisk fang.
The Cup was the target of the most audacious heist in wizarding history. Harry, Ron, and Hermione broke into Gringotts using Polyjuice Potion (Hermione as Bellatrix), Griphook as guide, and the Invisibility Cloak. They passed the Thief's Downfall, rode a mine cart, confronted a half-blind dragon, and entered the vault -- where everything they touched burned and multiplied, threatening to crush and incinerate them simultaneously. Harry grabbed the Cup with the Sword of Gryffindor. Griphook seized the Sword and betrayed them. They escaped on the dragon, bursting through the bank's ceiling and flying over London. Hermione destroyed the Cup in the Chamber of Secrets with a Basilisk fang. It was the moment she went from planning the Horcrux hunt to ending it.
Small golden cup with two delicate handles and a badger engraving. Protected in the Lestrange vault by multiplying and burning curses -- touching anything causes it to replicate and sear flesh.
Also known as: Hufflepuff's Cup, the Cup