Decoy Detonators are black horn-like objects designed to create a diversion. After being dropped, the decoy runs a fair distance away, makes a noise like a loud bang, and releases clouds of black smoke, diverting attention from the user.
Invented by Fred and George Weasley, they were for sale at Weasleys' Wizard Wheezes. Fred and George gave several of these devices to Harry Potter, refusing payment on the grounds that his thousand galleon investment had allowed them to start the business. 
Harry used one on 2 September, 1997, during the Infiltration of the Ministry of Magic, to create a diversion to distract several Ministry of Magic employees. He subsequently entered Dolores Umbridge's office to search for Salazar Slytherin's Locket, one of Lord Voldemort's Horcruxes .
Behind the scenes
- The Decoy Detonator makes an appearance in the film Harry Potter and the Deathly Hallows: Part 1. It serves the same purpose as in the book, but in the film the device is capable of multiplying itself to cause widespread confusion. However, it could also be assumed that this was a magical effect added later on by Harry or Hermione Granger, similar to Geminio.
- In LEGO Harry Potter: Years 5-7, both Fred and George Weasley have Decoy Detonators as controllable "pets".
- In the Magic Mischief' game in The Road to Hogwarts Sweepstakes, Decoy Detonators can be used to distract an enemy to continue in the game.
- Harry Potter and the Half-Blood Prince (First appearance)
- Harry Potter and the Half-Blood Prince (film) (Appeared on DVD 2nd-disc Special Feature)
- Harry Potter and the Deathly Hallows
- Harry Potter and the Deathly Hallows: Part 1
- Harry Potter and the Deathly Hallows: Part 1 (video game)
- LEGO Harry Potter: Years 5-7
- The Road to Hogwarts Sweepstakes