The cabinet that vanishes what you put in it
A vanishing cabinet in Borgin and Burkes has a partner at Hogwarts. Draco spends a year mending that pair so Death Eaters can walk into the castle.
Setup
1Harry Potter and the Chamber of Secrets · Knockturn Alley
Harry hides inside a black cabinet at Borgin and Burkes. He hears that the cabinet is almost unnoticeable, which at the time is only a place to hide.
2Harry Potter and the Order of the Phoenix · The Room of Requirement, used as a dump
Fred and George shove a student into a vanishing cabinet at school and he is later found jammed inside it. The cabinet is broken, and Montague’s accident is played as a joke with a nasty edge.
Payoff
Harry Potter and the Half-Blood Prince · The night Death Eaters enter the castle
Draco has been repairing the Hogwarts cabinet so that it connects to the one in the shop. When the pair works, Death Eaters step out of the Room of Requirement. The object Harry hid in, and the one the twins broke, are the two ends of the same passage.
