Impurity

are monsters that reside in Magano. They are the personification of the yin energy that gathers there. Any evil thoughts that any human may have the Ame-no-mihashira absorbs and seals into Magano. Soon after, the yin energy became dense enough that these Kegare began to form without will just a desire to destroy.

The Creation of Magano
Ryo Hyuga unknowingly created this hellish dimension when he entered Clairvoyance Burst Mode and used Six Paths: Ame-no-mihashira, attempting to trap Evil Mitsuki inside. Mitsuki escaped and after defeating him, Ryo discovered this dimension he had created. Ryo ventured to Hoshigakure to construct a dimensional gate that allows access to Magano. Sumaru agreed to allow Ryo to do it.

Sumaru's Idea
Soon, Sumaru had the idea of making Magano a training ground for the Hoshigakure genin. This setup was good as the genin could train on real enemies without harming their conscience as the enemies have no will just a desire to destroy. It was also good because Magano's miasma can affect people who aren't able to properly control the star chakra inside them so Magano could also be used to teach chakra control.

Lurking Dangers
Soon, Sumaru took note to the Kegare's increasing strength. He immediately opened a portal to the real world and took on the remaining Kegare himself. Midway, he met a Basara, a heightened Kegare. Sumaru then was pulled into a relentless battle between himself and the Basara eventually winning when he used Impure Legion Banishment: Divine Enchantment. He left Magano and repealed the law saying that Magano was a training ground because it is too dangerous. He forbid any entrance into Magano for any reason.

Elimination
Through trial and error, Sumaru realized that any normal attacks that just utilize chakra will not affect Impurities. Only attacks imbued with star chakra affect Impurities such as Yang Release: Impure Legion Banishment and Yang Release: Light-Warping Bullet.

Trivia

 * These are the main enemies in The Star Crusade along with Evil Mitsuki and his minions.