Modrons

Living constructs of geometric design, modrons are paragons of absolute order and largely alien to mortal comprehension. Since time immemorial, these enigmatic beings have retreated from the cosmos for centuries at a time only to reemerge periodically on an epic scale—tens of thousands of them gating in from their bizarre clockwork dimension to march a grand circuit across known existence. As planar scholars and doomsayers are quick to point out, the end of the next cycle is nearly upon us. If history is any indicator, a modron incursion is imminent, and woe to any who stand in its path.
Modrons are a physical manifestation of order. They have a decidedly clockwork appearance, their peculiar geometric bodies fashioned of gears, plates, and rivets forged from rare metallic alloys. Modrons are not wholly artificial, however. Living tissue is inextricably fused with their metallic exoskeletons. Their most disturbing feature is their eyes; great bloodshot orbs that stare uncaringly.
As living constructs, modrons incorporate both mechanical and biological components; the two are inseparable. Much of their fleshly being is vestigial and nonfunctional, but they retain many features of mortal creatures. The lowest orders of modrons are the closest to purely mechanical beings and do not require sleep; those above them in the hierarchy still need to rest from time to time, though they can go without sleep for long periods when necessary.
Some modrons continually strive to improve the efficiency of their race. These inventive beings are able to finely manipulate the latent power of The Clockwork Nirvana of Mechanus's energy pools to craft spells and devices unique to their physiology. Examples of such modron devices include mechanical limb extenders, magnetic clamps, and winglike appendages.

Absolute Law and Order

Under the direction of their leader, Primus, modrons increase order in the multiverse in accordance with laws beyond the comprehension of·mortal minds. Their own minds are networked in a hierarchical pyramid, in which each modron receives commands from superiors and delegates orders to underlings. A mod ron carries out commands with total obedience, utmost efficiency, and an absence of morality or ego.
Modrons have no sense of self beyond what is necessary to fulfill their duties. They exist as a unified collective, divided by ranks, yet they always refer to themselves collectively. To a modron, there is no "I," but only "we" or "us."

Hierarchy

Classification is a fundamental tenet of modron society. Modrons assign everything to category, especially themselves. Their society is divided into two primary castes: the base modrons, who are primarily laborers of low intelligence, and the hierarchs, who direct and plan. The base caste contains five ranks, from monodrone up to pentadrone, while the hierarch caste contains ten, including the singular Primus. 
A modron is permitted to communicate only with others of its own rank or of adjacent rank. This segregation is not the result of elitism, but a simple byproduct of efficiency. Most modrons are not even capable of comprehending the existence of higher ranks beyond their immediate superiors; likewise, those lower in the hierarchy than their immediate reports simply do not exist to them. For example, a pentadrone considers a decaton to be the ultimate form of its kind and cannot imagine anything greater, but when it looks upon a tridrone, it does not see a modron at all—nor can it even classify what it’s observing.
Thus, the very existence of Primus is secret to all modrons other than the secundi who directly serve it. Commands from the One and the Prime are passed down through the ranks, progressively translated into a form that the lower (and less intelligent) forms can comprehend. Whenever a modron receives instructions from a superior, it never suspects that those commands originated even higher up. From least to greatest, the ranks of the modron hierarchy are enumerated below.