Clues
The cup in the Lestrange vault
Hepzibah Smith shows Tom Riddle Helga Hufflepuff’s cup. He kills her for it. The cup spends the last book in Bellatrix’s vault, and Hermione destroys it with a basilisk fang.
Setup
1Harry Potter and the Half-Blood Prince · Smith’s memory
An old witch shows Riddle two treasures: Slytherin’s locket and Hufflepuff’s small golden cup. She is found dead soon after, and both objects are gone. The memory is how Harry learns the cup exists.
Payoff
Harry Potter and the Deathly Hallows · Gringotts, then the Chamber
Bellatrix’s panic at the word “sword” tells the trio her vault holds something of Voldemort’s. The cup is there. Hermione stabs it with a fang from the basilisk in the Chamber, the same kind of venom that killed the diary.
