The Shadow
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The Shadow (also called the Umbra, the Velvet Shadow and the Hisil) is a seperate plane of existence composed of ephemera and inhabited by spirits. It appears as a warped reflection of the physical world in which everything is alive. Anything of import that happens in the physical world may birth a reflection in the Shadow, and that reflection may live on long after the original is gone. Thus demolished buildings, ancient trees and extinct animals may all still exist there in spiritual form, as may ideas and concepts which pre-date anything in the modern world. The only exception are human beings and similar creatures, who are eerily absent from the Shadow.
The physical world and the Shadow are like two dark mirrors reflecting one another. Everything in the Shadow born from the resonances and forces of the physical world, and alterations to the Shadow causes subtle but potent changes to the essential character of the physical world. For example, a sudden surge in crime will birth a frenzy of spirits feeding on the fear, violence, and uncertainty, and then these spirits will seek to spread their resonances to better feed.
The worlds of flesh and spirit are kept seperate by a barrier known as the Gauntlet. Because of the Gauntlet, few mortals are aware of the Shadow; only spiritually-attuned creatures, most notably the Uratha, commonly interact with it and its denizens.