The Transmogrifian Torture (incantation unknown) was a curse which killed its victims. Professor Gilderoy Lockhart claimed that this was the spell to have killed Mrs. Norris before it was revealed that she had only been Petrified.
English "transmogrify", meaning "to change or alter greatly, often to grotesque effect", possibly implying that the curse changes the shape of the victim to cause pain.
Behind the scene
- Seeing that this curse kills, it is similar to the Killing Curse. The difference could be that this one causes torturous effects as opposed to outright death.