The Nine Nine Eighty-One Transformations was created by the Dark Crow during the Desolate Era. Once he became powerful, he rarely used it and just handed it to a junior. He used this art to escape from enemies countless times and to adopt a human form when he was traped in the crow body.
Nine Nine Eighty-One Transformations let someone to transform into someone else in a blink of an eye. The transformation is incredible perfect and nobody would be able to see through it, except a person with more expertise on the technique.