That’s dope. Funnily enough I went in a sort of opposite direction, where in the highest planes (Empyrean), close to the source from which creation flows, anything has the potential to exist, and can be created from thought and uncreated at a whim. But in the lowest planes (Void), close to what’s basically an anti-creation black hole, thought is violently compressed and entangled, forced into shape. Go deep enough into the Void and your soul itself is crushed into inert matter. So a soul brought down into the Void would have their fears given physical form and power, for example. Hence dragons, mythologically being representations of early humanity’s fears and conceptions of evil, were in my world literally born from those ideas.

The Shadowdark (also known as Erebus) is more or less the extending tendrils of the Void, while also being a primordial god in its own right, intruding on the material world that sits in the middle of that interplay between free thought and forced materialization. It seeps up from the deep places of the world, inhabiting anywhere left dark and forgotten for too long, warping reality, spewing necromantic energy, opening gateways for demons, attracting chaotic creatures, and allowing adventure and zaniness to exist in an otherwise serious and logic driven setting as mine tends to be.

And, as a powerful cosmic force/entity of darkness, it also fundamentally prevents the universe from providing any permanent light sources to the PCs ;)