If a wizard turns himself into an animal, he is endowed with the animal's brain, and starts thinking like the animal, the only exception being Animagi. Thus, casting this spell on oneself causes the loss of one's mental capabilities, and requires another wizard to cast an appropriate Untransfiguration afterwards.
Behind the scenes
- It is unknown if the spell was specialised to only turn humans into vipers, or if it could turn a human into any kind of snake.
- Wonderbook: Book of Spells (First appearance)