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.
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. All Onryō retain their abilities prior to Mental Breaking.
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. However one trait that all posses is forgoing meat that is not human, effectively making them cannibals. For reasons unknown, all Onryō do not attack animals unless they are attacked first, and even then they will avoid killing the animal. 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 40 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.
- 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. Not long after his transformation, he was killed in an ambush by Mumei. He became an Onryō after 5 years.
- Sanger Kazan: The second most recent Onryō and one of the most dangerous ones. His Onryō personality is an almost unnatural hatred for humanity. He became an Onryō after several decades with his Norowa Momōkugan awakened. He was later killed by his daughter Shirako after she became an Onryō herself.
- Shirako Yuki: The most recent Onryō a is considered just as if not more dangerous than Sanger. Her Onryō is described as almost childlike, treating killing as a game and acts as if she did not know what she was doing was wrong. Shortly after her transformation, she killed Sanger and implanted his Norowa Momōkugan into her right eye. She became an Onryō after an unknown amount of time.
- Some Onryō such as Norowa and Sanger have completely stopped speaking, only communicating in growls, while others such as Shirako are quite talkative.
- It has been theorized by a certain individual that it might be possible to control an Onryō with a powerful enough genjutsu. This seems to have occurred with Shirako.