An Aberration ( 怪異, Kaii) is a chaotic event that causes trouble to those it interacts with. When a certain item or living being comes into contact with a large concentration of Curse, they typically turn into troublesome beings that can throw their surroundings to a great turmoil. The most common aberrations are either living creatures like animals that became corrupted by the negative influence of the curse, or Amasogi that were born from the desires of a human being placed upon a certain object. Other less common types of aberrations include Tsugumomo that became a "Man-Eater" or a "Possessed One", human beings possessed by Amasogi.
The aberrations can be incredibly varied, and sometimes they might not be particularly dangerous but just troublesome. Often times however they can be powerful creatures born from a malevolent goal, resulting in them being very violent and could threaten the lives of those around them. If left alone aberrations, especially those of higher peligrosity, would be an extremely destructive and chaotic natural disaster so, in order to deal with them, specialised groups of people called Exorcists were born. They exorcists are gifted warriors, capable of eradicating aberrations while remaining unknown to the common population, and they're in charge of deleting aberrations under the guidance of temples like Tsuzura Temple.
The temples determine how dangerous aberrations are by grading them through "Kyus", with 9 being the lowest priority, weakest and least dangerous type and increasing from there the lower the number is. Above all these there is also a special type known as "Frozen Stock", which are unique aberrations that achieved an extremely high level of power but are considered of low priority due to different circumstances, like living on secluded areas or not being violent entities. These are often left alone unless an exorcist decides to deliberately hunt them down, like it was the case of Kanaka Kagami.