Mechanus

Mechanus is one of the outer planes that exists to balance out the chaotic energy that is Limbo. However, Just as Limbo creates madness through sheer chaos, so does
Mechanus with its unnatural sense of order. For every degree of hot, there is one of cold. For every inch of up, there is another of down. The plane is ruled by none but the collective conscious that the plane exudes itself. In some ways the plane is a living and thinking body that works at all times to make sure that everything is an efficiently working machine. The entities that dwell there, the Modrons, are a collective hive mind that are all built for a purpose. Each understands what they are made for and will do whatever is needed of them to keep the balance. For everyone there is another.
No deity will ever be able to claim it as their own because without the constant balance, the plane will begin to tear itself apart. The land and structures found here are infinitely spanning and massive sets of gears that are at the smallest 1,000 miles in diameter. While the expanse is horizontal, each gear has another just as large as below it. The space between the two is filled with flowing air making flight very easy but still possible to fall though. Gravity works on both sides of the outside of the two and they all move clockwise in sync. What is on the surface of one be it a mountain, building, or expanse of terrain, so too will be on the bottom.
If a new being were enter this plane, temporarily disrupting the perfect balance, chaos will ensue for a brief time. While the plane enters a moment of brief absolute chaos, it will begin to construct a new entity. This new creature will be the exact opposite in various ways to the new being. Upon initial entry to the plane, neither the Modrons nor the plane itself will consider it hostile. They will not find any hospitality or hostility; they will simply be. If the new creature that enters leaves, the plane will reenter its state of pandemonium unless the opposite copy also leaves. Simply killing either will not work because there will still be a body obstructing the perfect harmony. It must be expelled.