Impurity

An Impurity (or "impurities", also known as "kegare" in the anime) is the name for monsters that reside in a world called Magano, but at times they break into the human world to abduct and kill humans. They're the main enemies of the Exorcists.

Origin
It is unknown when and where they comes into existence. It is said that Impurities exist since the beginning of time. Futhermore it is said that they are born from the depths of people's sins and karma.

Appearance
Through the sheer amount Impurities, they each vary on their appearance and size. But they all share specific characteristic traits like the dark skin and a net/number sign on their chest or stomach.