Mechanus
"Justice is incidental to law and order"
- – J. Edgar Hoover
Mechanus (known in pre-Planescape times as Nirvana, with both names later being combined into The Clockwork Nirvana of Mechanus) is the Plane of Lawful Neutral in Dungeons & Dragons and its Great Wheel cosmology, introduced to the multiverse in the Advanced Dungeons & Dragons setting Planescape.
Representing the pure perfection of order; order over everything else. As the pinnacle of structured thought and action, Mechanus appears as an infinite array of clockwork machinery. In this vast plane-machine, life clings to gears the size of continents and pistons the size of planets, completely in tune with the ticking of the multiversal clock.
Unlike nearly all other planes, Mechanus has only a single layer, albeit one made of many large and intricate parts. "Regulus" is the name of the cog(s) where the Modrons reside and are manufactured. "The Center" is literally the centermost cog and its turning turns all others. Unfortunately, it's overrun with Formians and therefore largely inaccessible to anyone else.
Mechanus's most iconic inhabitants are the Modrons, geometrically shaped extraplanar beings fashioned of otherworldly machinery. Come third edition the Modrons lost their place of prominence and were supplanted by the Formians and the Inevitables, something explained officially as being due to the death of Primus at the hands of Tenebrous (the undead Orcus), as detailed in the adventure "The Great Modron March". Unofficially it's held that Modrons were pushed out of the limelight because they were seen as embarrassingly goofy by Wizards of the Coast. Fortunately, like other goofy old-school things, they were rescued by 5e, restoring them to their full, somewhat silly, glory.
The Mechanatrix race is the Mechanus-descended equivalent of Aasimars and Tieflings, being genetically something like 99% human and 1% steampunk robot.
There is a near complete absence of Lore on the petitioners (the D&D word for departed souls) who wind up on this plane, almost as though Big Brother transcended the Narrative Barrier and is actively censoring the info. Although considering that it's very difficult to invent any such info oneself, it's quite possible that the writers can't either and are jumping through hoops to avoid having to bring up the subject. Because on the very few occasions where they do, they either give a description that make no sense for the plane's alignment (such as a social darwinist society where those at the bottom are ground up to be made into Modrons), or else give incomplete info (explaining that they're unremarkable in appearance and behave nicely when you obey the rules but throw fits when you don't, while completely neglecting to describe what exactly they do all day).
Pathfinder's analogue to Mechanus is Axis.
