Astral Plane

The astral plane acts as a mirror sea to the universe above it, though each realm is actually too high up to be viewed as anything but a star-like object. The astral sea works as a mechanical conveyer belt for the destroyed parts of a realm. Dead bodies travel here, though souls cannot pass through the membrane of their respective plane.
The astral sea is like the nitrogen cycle, with the sea as a decomposer that returns important parts of the realms to rebuild, build or destroy planes.
The temperature is mild, but not comfortable. Every 60 seconds in the plane, air grows heavier and a DC20 wisdom save must be made to not try and lay down in the astral sea, like the pull to be consumed by the Steppe.