The Grind , not to be confused with "Grindr" ,though pretty sure I've called this place Grinder a few times in the other texts somewhere. It's modeled off Acheron which I think was Lawful Neutral Evil but honestly you could make a case for it being near the Chaotic planes too.
Canonically in planescrap it sits between The Tower of Babel and The Heart of It All but also drifts out over the Plains of Relevation.
This one basically got done. Monsters, Distinctive & Vast Spectacle, Weird Ecologies and Economies, Valuable Goods, Unique Hazards, Rules for Exploring it, Encounter Tables, and Art. Even got an Appendix N at the end.
Here's the links to the planescrap primer: