Shadowside is kind of just a mess. On the one hand, you have Aphelion, which is a nice little round world. It's chilling in the aether, aka the cosmos, which is where the sun and moon live. Those are the eyes of the dead god who created the universe. Well part of that universe, maybe, is Shadowside. It doesn't exist in the aether, you can't see it from Aphelion no matter how hard you look. It's thought to exist "behind" the sun and moon, both in the sense that they do not shine there, but also that it was not created on purpose. At least one of the old gods claims otherwise, but everyone thinks he's lying or talking nonsense (which he tends to do).
Shadowside cannot be reached from Aphelion through non-arcane means. If that wasn't enough, there's a cosmic barrier intentionally placed between the two realms to make passage just hilariously hard to do. It's easier to get to Shadowside from Aphelion than the other way around, which is entirely because of the aformentioned barrier. That part's on purpose. People travel anyway.
As for the realm itself, it exists as a shadow cast by the real universe. It's chaotic, poorly curated, and splintered into dozens, perhaps hundreds of small worlds that can only interact, again, by arcane means. Some of these worlds seem more grounded in reality and design, and wouldn't be unfamiliar to the natives of Aphelion. Others struggle against the tethers of time and reason. These worlds blink into existance in medias res, and the most chaotic will blink back out as suddenly as they came. The locals, if any, will claim no knowledge of a genesis or "beginning time."
The three elder gods do not tread in Shadowside, as their divine abilities are bound to their own part side of the universe. Though they are perfectly able to travel, they seem to fear retribution from the realm itself for being abandoned.
Shadowside may or may not be connected to the Fourth God.
I should compile all the lil shadow worlds I make