The Shadow Realms is the place you go where you die. No matter if you're in heaven OR hell OR limbo, this place contains all of them, you just can't see it since it's invisible.
The Shadow Realms mainly comprises of absolute opposites, like Heaven and Hell, or Black Holes and White Holes, or Pitch Black and Pitch white. Also, this "verse" has an invisible cloaking on it that makes it invisible to any non post-realm creature, such as The Inverse, who doesn't know of this verse's existence, just like The Outverse. Also, this verse is dark, even if you can see it. In fact, it's so dark, that even Vantablack or Black 3.0 come nowhere close to how dark this is. This realm absorbs 100% of light, meaning that not even The Light Realm can bright it up.
Some people think that the shadow realm absorbs all light because it has a giant Shadow Hole inside of it, which absorbs all light near it, causing it to not be seen any more, and sent to The Light Realm or some place similar.