Stagnant souls (ヨドミ, yodomi, lit., "stagnant") are souls of dead people who remain in the Threshold, awaiting purification. During their stay in the Threshold, they manifest their wishes and live quiet lives, although not everyone would share the same experience. Some would transform into corrupted souls and these souls, if left unattended, can endanger the lives of other stagnant souls and upset the balance of souls in the Threshold. Because of this, Black Stars were created to answer to these threats and purify the souls using their weapons.
All stagnant souls, when dormant, take a spherical shape, with "flames" surrounding it. When active, stagnant souls may take on human or animal forms depending on their previous life, although stagnant souls with animal forms are a relative rarity. These souls contain the memories of a particular person who died in the living world, and they alse retain the wishes of that particular person, alongside his or her lingering regrets or grudges in life.
However, when a stagnant soul reaches a particular degree of corruption, which happens when the soul is consumed by negative emotions such as anger or disgust, the soul becomes corrupt, transforming into a more hideous form that reflects the degree of that soul's corruption. One important manifestation in a corrupted soul is the emergence of weapons, which make it a threat to other stagnant souls when left unchecked.