Lawful neutral paramortals inhabiting the clockwork plane of Mechanus
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The modrons are the ultimate embodiment of law and order. As such, they see order as the be-all and end-all of existence. To them, all life must spring from a pool of logical action: one's superiors. There is no individuality within a rank; all modrons refer to themselves as 'we'. They continually strive to make Mechanus as perfect as it can be, and if given the opportunity, would extend that law and logic to the rest of the planes.
The modron hierarchy reflects the race's strict and unbending order. Modrons have a difficult time acting outside of a certain set of actions, set by their form and function. When they encounter something that they can't deal with, the automatic reaction is to ask for orders from a superior modron. In addition, each modron is unaware of the existance of any other modrons beyond the ones one rank above, and one rank below it. If a monodrone was shown a tridrone, it could not even say what it was looking at. Thus, orders and information must travel up and down through the ranks, step by step.
At the head of the modron hierarchy is Primus, the modron 'god'. Below it are four secundi, sixteen tertians, and so on. At the lowest ranks, there are uncounted numbers of small, spherical monodrones. Each time a higher-ranked modron is killed, a modron of the rank below it automatically transforms to take the place of the dead modron. At the lowest rank, the monodrones reproduce by fission.
On the plane of Mechanus, the modrons are almost omnipresent. It's hard to go more than an hour without seeing one of the geometric-shaped paramortals. They run their realm, Regulus, with precision and a enormous body of rules and regulations. Travellers to Mechanus are advised to pick up the 300-volume "portable" law summary.
However, the modrons' perfection is hard to maintain. Sometimes, though some accident, a modron will gain a sense of individuality. These entities, known as 'rogue modrons', are hunted down and destroyed by the other modrons.
Much about the modrons is mysterious and unknowable. But the most apparent aspect of this is known as "The Great Modron March." Every cycle, a huge 'parade' of modrons makes their way around the Great Ring, passing through each plane. Very few survive, if any at all -- but the modrons keep on with the march at the regularly spaced intervals.