Onryō

An Onryō (怨霊, Onryō) is someone who possess the Norowa Mōmokugan and has gone through a process called Mental Breaking. It is a title given to someone who has gone through the Mental Breaking process and has therefore "lost" their humanity.

Norowa Mōmokugan
The Norowa Mōmokugan is the evolved form of the Mōmokugan that is unlocked in dire circumstances and grants the user a powerful ability. While it gives the user great power, the Norowa Mōmokugan is inherently unstable and after a number of years or decades will begin producing a special chakra that alters the user's personality and to an extent, their body chemistry.

Overall, Onryō can be described as monsters, characterized by their desire to kill humans, and inability to feel pain. An Onryō's specific personality varies between individuals. The process to turn into an Onryō is not gradual, but very sudden, with the transformation itself taking less than a day. The amount of time a person has before transforming also varies even between those who have had the same power as others, with the amount of time taking anywhere between 5 to 30 years. It has been noted that once the Norowa Mōmokugan is unlocked, there is no way to avoid Mental Breaking, not even by removing the Norowa Mōmokugan.

Known Onryō

 * Norowa Hikage: The first user of the Norowa Mōmokugan and the first Onryō. His Onryō personality was described as being similar to a rabid animal.
 * ???: The most recent Onryō and arguably the most dangerous. His Onryō personality is an almost unatural hatred for humanity.

Trivia

 * While it is rare for them to do so, Onryō can still speak, though it is not possible to reason with them.
 * It has been theorized by a certain individual that it might be possible to control an Onryō with a powerful enough genjutsu.