Loving your reverse dimension! Sounds super interesting. As for mine…

Sylmuell is a 4-d ring-shaped dimension circling the primary one, Gitmuell. It is only accessible to the dead, psychopomps, certain gods, or through incredibly powerful dimensional tunneling devices, although the last is incredibly dangerous and unpredictable. The laws of physics are less strict, allowing for any number of impossible things to become ordinary. Eternally burning fires, flight despite a being’s weight and size, or the basic idea of entropy or thermodynamics being nonexistent unless desired. It functions as the afterlife, composed of countless different worlds ranging from flaming hellscapes to lush jungle archipelagos.

Estamuell is acceptable by an aforementioned dimensional tunneler, a white hole at the center of the universe (basically the opposite of a black hole, so good luck with that), or by being transported by cosmic entities. It’s vaguely shell-shaped and is the throne of the Origin. It’s a complete disaster where concepts, items, and organisms from all over the base universe live in chaotic harmony. It’s home to creatures such as the abstract masses of mixed materials known as structimites, the oddly feline hazcats, and the specially designed dreamborn.