The Subplanar Realm
The world just beyond the world. The underneath and the behind.
The subplanar realm is an entire ecosystem of entities, some call them spirits. But by and far most of them are not related to the living in most ways.
Most of the entities, called by the Wardens maiar, are similar to what we may call bacteria, fungi and insects. Compared to even mushi they are orders of magnitude smaller.
- Like krill(maiar) compared to a blue whale(chickadee's soul).
The second largest domain of entities of the subplanar realm are mushi. As Warden Philosopher Uzak said:
"If four of your fingers were animal life, and your thumb was plant life then humans would be at the tip of your middle finger. This would be the furthest away from the heart. As you traveled down your fingers to the wrist, he said that the arteries and veins start to "connect" and this would be fungi/bacteria. He then moved all the way to his heart and said that this was what mushi were."
Unlike maiar, or most souls, many mushi reside within the veil between the living realm and the subplanar. Ofttimes entering into the living realm itself. Those that reside in these places are seemingly magical creatures which make their home in rural areas, and most of them depend upon either forest or human life in order to live; this is why they are considered a form of parasite.
The purposes of the mushi range from helpful to harmful to downright malicious, but it should be noted that mushi are not evil, even though their effects are dangerous; they, like most beings, are simply trying to live. This is emphasized by Uzak, who dedicates his time to helping people and mushi live in harmony, instead of destroying mushi like most conventional mushi masters do.
Souls are by far the dominant form of life in the subplanar realm. If maiar are kingdom Archaea/Archaeabacteria, mushi are kingdom Plantae, Fungi, and souls are kingdom Animalia.