The diadem on the bust
Rowena Ravenclaw’s diadem is a Horcrux sitting in the Room of Requirement. Harry has already used it as a marker for his Potions book, without knowing what it was.
Setup
1Harry Potter and the Half-Blood Prince · Hiding the Potions book
Harry stashes the Half-Blood Prince’s book in the Room of Requirement and drops an old tiara onto the bust beside it so he can find the spot again. The tiara is furniture to him.
2Harry Potter and the Deathly Hallows · Luna and the lost diadem
Luna takes Harry to Ravenclaw tower. The house’s story is that Rowena’s diadem was lost after her daughter stole it. A ghost confirms the daughter hid it, and that a young man later charmed the story out of her.
Payoff
Harry Potter and the Deathly Hallows · The Room of Requirement on fire
Harry remembers the tiara on the bust. It is the diadem, and it is a Horcrux. Fiendfyre in the room destroys it while he, Ron, and Hermione are still trying to get out.
