Within the Astral, the Plane of Thought, lie the physical representations of every concept which underlies the psyche of thinking creatures. Just as Limbo is a plane of ultimate Chaos - a place where nothing is consistent from one moment to the next, and all is made of formless chaos-stuff - Mechanus is the plane of ultimate Law. Mechanus is called the clockwork plane, and with good reason - after all, the plane itself is made of enormous gears which turn constantly under some unseen power. For most visitors, this is all that there is to be seen of Mechanus, and many go away thinking it is a boring and homogenous place. There is much more to it, however. It is a place of pure causality, of ultimate unfeeling order. It is a place where prophecies are born, where things are preserved in stasis for all eternity.
At the edges of Mechanus, it simply appears that you have arrived at a very peculiar and regimented place. Here, massive clockwork cogs are home to the less strict denizens of the Plane of Law. Whether the domain of Lawful gods or simply towns and kingdoms for planars and petitioners of a Lawful bent, even these places will seem unforgiving and obsessed with the proper way of doing things to foreign visitors. But their Lawfulness is of a more mortal kind; it is bureaucracy, a desire to see that things are done properly, or a steafast dedication to certain principles. In this way, the denizens of the endless clockwork are very different from the modrons that inhabit the deeper parts of the plane. These beings are concerned with universal Law; their duties are directly related to the function of the great machine that is Mechanus. The closer one gets to the center, the more alien the plane's denizens become.
When attempting to discern the function of Mechanus, one must consider Primus; without Primus, there would be no Mechanus. Without Mechanus, the multiverse would be left to the mercies of Chaos. Primus would say that Mechanus truly is the center of the multiverse, whatever misguided scholars may say about Sigil. From the very center of Mechanus flows the energy that turns each and every gear. Perhaps the physical gears of steel and other metals stop at the edge of Mechanus, but the great mechanism of the multiverse continues far beyond its limits. Down to the smallest grain of sand on the most inconsequential of Material worlds, everything in the Multiverse is driven by the great machine.
This is another way of saying that the gears of Mechanus are the Astral representation of the influence of Law in the multiverse. They are the physical manifestation of causality; their turning is what causes time to flow, what makes one event follow on from the next. Because of Mechanus, creatures live only forward in time. Because of Mechanus, fire burns; light travels; heavy things fall, and light things float. And as Primus is the autarch of Mechanus, he is the conduit for the power of Law. It is not surprising, then, that he is worshiped - and more often cursed - by a different name: as Time, which is inevitable and waits for no man.
In the exact center of Mechanus is an massive metal tower, about a mile across, that extends infinitely upwards and downwards. It is made of a substance that seems like metal, but close examination would reveal that it appears this way only because one lacks a frame of reference. It is perfectly smooth, cold to the touch, impossible to break or mar, and nothing will adhere to it. It is the essence of stability, and it revolves endlessly - slowly, but inexorably. From the Mainspring, which draws its energy from an unknown source, thousands of shafts, gears and cogs are connected - by extension, all the gears and cogs of Mechanus are powered by the Mainspring's turning.
In the center of the Mainspring there is a spherical pool of radiance. Within that pool dwells the one known to the modrons as Primus, the Autarch of Causality. Although he spends most of his time immersed formlessly in that pool, he can take physical form if there is need of it. Within his hands, all causality exists. He is as powerful as any god; more powerful than most, in fact. Nonetheless, his hand is rarely moved. The multiverse is a great machine; the wheels of that machine can turn no other way than the way they do, and he sees no need to interfere. Only for the sake of the modrons and the functioning of his perfect machine does he stir himself. Even this, however, he does through intermediaries. Such a being of rigid law he is that acting directly is dangerous; more often than not, it results in his own destruction. The most common way in which he extends his power is in the form of the priest spells granted to modrons.
Fortresses of the Secundi
The Mainspring has only four entrances; these are located in the heart of four great fortresses that are built around it. Each of these fortresses is the home of one of the Secundi, the highest-ranked of all modrons. It is because of the Secundi that Primus' destruction does not mean the end of the multiverse; Primus is merely first amongst modrons, a symbol of the power of Law. As long as the modron hierarchy remains intact, there will always be a Secundus ready to take on the mantle and become the new Primus. However, the interregnum creates great turmoil in the multiverse, and the power of Chaos invariably grows while the new Primus tries to return order to the modrons.
Outside of these rare periods of turmoil, the four Secundi rule in perfect harmony with each other. Created by the new Primus upon his ascension by promoting a lesser modron, they are ultimately responsible for overseeing the entire quarter of Mechanus that falls under their purview. This includes the Mechanism, the Great Gears, and even the Currents of Causality. Although almost everything is delegated to inferior modrons, Secundi often give direction to those below them - ordering a new part of the Mechanism to be constructed, passing down edicts to the denizens of the Great Gears in their demesne, or even making orders related to the Currents of Causality.
The Secundi do not dwell in their great fortresses alone, however. Besides being their homes and the entrance to the Mainspring, the fortresses of the Secundi are also the location of the courts of modron law. Each Secundus is attended by a number of Tertians, who run their courts. In turn, these Tertians supervise the Quartons below them and are responsible for hearing all crimes brought against the rigid orthodoxy of the realm.
Outside of the Mainspring lies the most complicated and sophisticated part of the machinery of Mechanus; the Mechanism, which is the primary home of the modrons and maruts. Unlike the Great Gears upon which cities and kingdoms are built, the Mechanism is immensely, almost absurdly complex; a labyrinth of gears, many-faceted contraptions and ingenious clockwork. It is in the Mechanism that the raw power of Mechanus is used to power the mind-boggling machinery that causes the natural laws of the multiverse to function.
The Mechanism is incredibly intricate, and its many different moving parts are dedicated to all manner of different aims - though always with the overarching objective of enforcing and maintaining the natural laws of causality. It is maintained and inhabited almost entirely by the modrons. A few examples of sections of the Mechanism include:
- The Menageries, which are in effect enormous museums dedicated to magically preserving specimens of the creatures and phenomena found throughout the multiverse. This magic, driven by the turning of the Mechanism, is similar to a Temporal Stasis spell. Some of these areas are less like museums, though, and more like prisons - built to house creatures too chaotic and dangerous to be let loose in the multiverse.
- The Edifices, which are delicate arrays of sensitive equipment that can feel the slightest tremor in the natural laws of the multiverse. When a grave violation of the natural order is made, it can be detected by the maruts who man these outposts. It is then their duty to go forth into the multiverse and either right the wrong or punish those who transgressed against the order of things.
- The Apparatus, which are measuring devices maintained by the modrons. They measure almost everything which can be quantified, even seemingly nonsensical things - the number of birds on Greyhawk, how much precipatation the moon has received, and the number of wars started over land disputes in the last 6 hours, for example. The attendants of these devices frequently sally forth to find the parts they need to maintain them, or to perform field tests for the purpose of calibration.