The Shadow
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. It stands as a warped reflection of the physical world, pulsating with vitality and endless hunger and resonating with the echoes of the material realm.
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. 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 relationship between the physical world and the Shadow resembles that of two dark mirrors, eternally reflecting each other's essential nature. Everything in the Shadow is born from the resonances and forces of the material realm and alterations within the Shadow yield subtle, yet potent changes to the essential character of the physical world. A sudden surge in crime, for instance, births a frenzied multitude of spirits that feed on fear, violence, and uncertainty, and in turn these spirits will seek to spread their resonances to better feed.
A metaphysical barrier known as the Gauntlet acts as a demarcation between the worlds, keeping flesh and spirit separate and obscures the perception of the Shadow from mortals. Only spiritually-attuned beings, most notably the Uratha, commonly interact with the Shadow and its denizens. They possess a connection to the supernatural that allows them to traverse the subtle intricacies of the Shadow, communing with the spirits that dwell within its alien depths.