The well of possibility

As with any system of reality that has realms connected by portals, especially ones with individual cosmos, there has to exist something in between.
This is the well of possibility. Home to fey and gods alike, the well is a void of things outside the comprehension of 3d space and time.
The well itself, if viewed by conventional eyes, is a fractal of lenses in the truest of senses - invisible breakages of light passing through and being reflected across increasingly smaller and repeating patterns and waves. 
This does not produce the cacophony of light one might expect, but instead an impossible distortion of things barely infront of you.  With no true matter native to the well, this means that naturally the well would be in a state of eternal light. Except there are those portals we talked about.
In the well the portals cut impossible curls of darkness, where once would be spirals of insane color in spectrums unknown to man. Now only darkness remains, the pathways severed but filled with a nothingness that consumes the potential native to the well.
The beings still native to the well resent the inhabitants of the realms, giving rise to creatures that ooze from the gaps between well and realm - shadowkin. 
Shadowkin are believed to be from Infernus but are actually native to the well and feed on the negative emotions of those they parasite to. Forming as a shadow to their host and a sigil on their skin, the shadowkin encourage distrust in the mind of those they attach to and feed on the result.
The well contains moments of fracture, allowing things outside this reality to sink in, or be sent out - like holes in a boat, or straws through paper.

by Kranjax via Midjourney

(A crack in reality)