The Transmogrifian Torture was a curse which Gilderoy Lockhart suggested could have killed Mrs. Norris (before it was discovered that she had only been Petrified). Lockhart claimed to have witnessed the spell more than once and stated that he knew its counter-curse.
Since Lockhart based his entire career on lies, it is unknown if the Transmogrifian Torture is a real spell, or simply something which Lockhart made up to make himself look like an expert in the Dark Arts.
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.
- Harry Potter and the Chamber of Secrets (First mentioned)