The Thief's Downfall is one of the many defensive enchantments employed at Gringotts Wizarding Bank.
The Thief's Downfall has the appearance of a waterfall, which falls directly over the tracks used by the mining carts of Gringotts. Its purpose is to wash away any concealment or enchantments (including the Imperius Curse) that are being used by the occupants of any mining cart that runs through it, as well as causing the cart to derail and crash, preventing the thieves from progressing any further. However, it only seems to affect magic that had been cast upon something else that the waters have run upon, since Hermione was able to cast a Shield Charm directly at the waterfall, re-directing its running waters elsewhere.
During their 1998 break-in at Gringotts when they attempt to access the Lestrange vault, Harry Potter, Ron Weasley, Hermione Granger, and Griphook the goblin encountered this spell. As observed, the waterfall removed all the effects of the Polyjuice Potion from Hermione, a number of transfiguration spells from Ron, and lifted the Imperius Curse Harry had placed on Bogrod the goblin. Furthermore the water knocked the cart off course and, were it not for a Cushioning Charm Hermione produced at the last second, all the occupants of the cart would have almost certainly been killed.
Behind the scenes
- The Thief's Downfall had no effect on the Invisibility Cloak worn by Harry. This was probably something to do with it being the Cloak of Invisibility (a Deathly Hallow rather than an ordinary Invisibility Cloak).
- It also spared Hermione's beaded handbag, which would have probably burst should Hermione's Undetectable Extension Charm be lifted. This may have had something to do with the "Undetectable" part of the charm, which hides the charm itself. Therefore, it is only logical that if Thief's Downfall did not detect the charm it would not have lifted it.
- In the novel, the cart derails after passing through the waterfall. In the film, the cart slows to a stop, an alarm is activated, and the cart's seats drop out from under the passengers.
- Carts probably derail after running through Thief's Downfall because the carts themselves are probably charmed to both keep the cart running until it reaches its destination and keep the cart rooted to the rails and the occupants inside the cart despite the many high-speed twists (some of which invert the cart briefly) it runs through.