The Ethereum

The Ethereum is the plane of existence which is superimposed upon the Material Plane. Creatures live within it and can traverse through the realms, and creatures within the Material Plane with the ability to do so may do so freely as well. However, both environments are seemingly hostile to one another. There are two main parts to the Ethereum, the Border Ethereum and the Deep Ethereal.
Much like their names suggest, the Border Ethereum is the transient layer between the Deep and the Material Plane, acting as shallow waters for anything moving in between. The Border Ethereum is where the most "normal" of the creatures exist; creatures comprehendible by the human psyche. While still terrifying, their visages can be seen and the creatures fought.
The Deep Ethereum is the farthest layer from the Material Plane, and for good reason. Eldritch nightmares and horrors beyond human comprehension travel amidst this layer, seemingly held at bay by the Border Ethereum. They move amidst this layer, inducing near permanent madness into anyone who delves too deeply into its secrets.
The Ethereum is the plane of existence most avoided by scholars. Shadowy images of the creatures tug at the corners of your vision, while whispers of madness find their way into your minds. Many-eyed and many-limbed creatures watch, hanging off buildings and clinging to walls like demonic spiders. The knowledge that these creatures are always there and always watching proves too much for the sanity of most people.
There are stories of those cursed with Sight, the ability to constantly see into the Ethereum. While most succumb to insanity, there are those who find their home amidst the madness, telling tales of the creatures and of the Deep Ethereum that are used only in nightmares.