Tortures, eventually to death
The Transmogrifian Torture (incantation unknown) was a curse that which presumably tortured it victims to death over time. It is different from the Cruciatus Curse in that it can kill. Professor Gilderoy Lockhart claimed that this was the spell to have killed Mrs. Norris before it was revealed that she had only been Petrified.
Transmogrifian comes from the 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 eventually kills, it is similar to the Killing Curse. The difference could be that, like the Cruciatus Curse, this one causes torturous effects as opposed to outright death.