Modron

A modron's body is made of a unique metal called Modron Brass not found anywhere else in the planescape. They have a variety of body plans and configurations, but their limbs are largely very thin and flexible, with multiple articulated joints.

There are five known subtypes of modron, with each individual of a given shape identical to all of the others. These subtypes all share the same automaton-like metal frames, but otherwise differ greatly in body plan as well as ability.  

Monodron

Monodrons, the simplest modrons, are one-foot-tall creatures consisting of a mostly smooth metallic sphere with one central eye, two spindly three-fingered arms, two small feathered wings, and a single short, jointed leg. They are the only modrons light enough to be capable of flight. Monodrons have intelligence similar to many birds; they can communicate or mimic simple messages up to twenty-five words or perform basic tasks. They are by far the most common type of modron.

Duodron

Duodrons directly supervise 55 monodrons each. Their bodies are two-segmented regular hexagons that can fold themselves into a trapezoid-like shape down a central vertical joint; as their eyes are on the inside of the shape, this does render them temporarily blind, though it increases their defensive abilities. Two of their limbs have five-fingered hands, and the other two have solid, boot-like feet at the end of flexible ankle joints. When fully unfolded and standing upright, they are two-feet-tall. Armed duodrones wield simple javelins. They have the intelligence and mannerisms of a herding dog, and can perform two simple tasks at any given time, as well as communicate in messages of up to 144 words.

Tridron

Tridrons supervise and direct 34 duodrons each. They are pyramid-shaped, three feet tall, with six legs and three arms as well as an eye on each triangular face. Tridrons are the modron most likely to act as sentries for danger; for this reason, the Erdelan Intelligence Agency uses an emblem of a tridron on its badge. They are also the modron most likely to lead modron forces into battle against the forces of chaos.

Quadron

Quadrons are five feet tall and shaped like a perfect cube. One limb extends from each of the cube's vertices; their legs and arms are all identical and interchangeable with fully opposable fingers, and they are able to stand upright, crawl like spiders, or climb vertical and upside-down surfaces with ease. Quadrons have the intelligence of an average adult humanoid, though they have much more difficulty with spontaneous problem-solving. They are, however, capable of archery and artillery in multiple directions, as they are able to rotate their six eyes to any face of their cube at will. Quadrons control 21 tridrons each.

Pentadron

Pentadrons, the most complex modron ever observed, have the largest array of moving parts and configurations. They can defend themselves by folding up into a solid convex dodecahedron with pentagonal sides, or unfold into arrangements of up to ten limbs whose functions can vary at will. Most commonly, their outer sides unfold into five larger, more powerful arms with flexible plated joints, while they support themselves on five taller but thinner legs. They often reverse this body plan when needing to hold fragile objects or move quickly and quietly. Pentadrons stand exactly 8 feet tall when fully unfolded and standing upright, and have an 8 foot wingspan from the tips of their largest arms. Pentadrons synthesize information and adapt to new situations in a way that demonstrates intelligence similar to an above-average humanoid; they are even capable of learning lower-level spells such as Command and Zone of Truth. They are also the only known modrons that exist simultaneously on their current plane and the Ethereal Plane at all times; some sages consider this a form of fourth-dimensional existence. Each pentadron has direct command over 13 quadrons.

Higher Orders

Several independent accounts have verified that multiple pentadrons exist in the multiverse at one time, though the precise number is unknown. This implies the possible existence of hexadrons or even beings beyond. However, these have never been observed. Some theories speculate that higher-order modrons exist in five or more dimensions, in a way that renders them unobservable to mortal eyes.

Modrons do not seem to reproduce independently via any method, and the majority of modrons sprout and are born in a single simultaneous harvest in Arcadia. If a modron is killed anywhere in its life cycle, one modron of each subtype under it is immediately promoted, and a new monodron identical to the rest of the current crop sprouts at the center of the orchard. This lone monodron immediately attempts to join the march via the most direct possible route; they are occasionally caught in reactive eddies of chaos magic and temporarily shunted to an unrelated plane.

Modrons begin and end their life cycles in the perfectly flat fields of Arcadia. In a mandala-like orchard somewhere in its depths, identically-spaced sprout fruits in the shape of perfect geometrical solids, which ripen into modrons. All at once, each crop of modrons drops from its mother plants and begins its great march across all lawfully-aligned Outer Planes, criss-crossing over the Liminal Plane and Astral Plane repeatedly over the course of the next seventeen years. At the end of their journey, the modrons ascend into the skies of Arcadia. Their automaton-like bodies dissolve into clouds, and they fall across the plane as a shower of precise, rhythmic rain.

Every seventeen march cycles (289 years Locumtempus Materiae), the orchard sprouts seventeen times as many modrons as usual. This larger march sprawls well beyond the modrons' typical territory, cycling through all known corners of the planescape. These larger marches have historically corresponded with major events imposing greater order on the universe, from the establishment of Material cities, kingdoms, and regimes, to scientific and arcane discoveries, to great retreats of demons in the Blood Wars and even the final verdict of the Litigation of Flames. Some cosmologists theorize that the greater presence of the modrons directly causes the imposition of order on the universe.

Modrons have never been observed eating or drinking water, and they seem not to need air to breathe, as they survive perfectly well in the Elemental Planes of Water and Fire without needing additional breathing apparatuses.

Any type of modron can communicate only with the order of modron directly above or below it; their gaps in intelligence seem to be too great to understand each other beyond that. Modrons, even higher-order ones, do not seem capable of grasping the concept of an individual self or ego.

Modrons have a eusocial societal structure, similar to kobolds or bees. Each type of modron fulfills its role and its role alone.   Modrons communicate with each other in a unique code of clicks and monotone beeps with highly regular syntax and "word" order, but little to no complex abstract expression. In recent years, this "language" has been studied with great interest by artificers, who believe it may have future applications in nonmagical long-distance communication. All modrons are able to mimic the sounds of Common and other simple Material Plane languages, but only tridrons and higher seem to be able to understand it.

The conjurers and artificers of the Age of Complexity have studied modrons with great interest, when capable of capturing them. The spells Sending and Unseen Servant are heavily influenced by the abilities of the monodron. Further developments are in progress, particularly attempts to synthesize the Modron Brass of which they are made.

All modrons possess Truesight, which renders them immune to the power of illusions. Quadrons and pentadrons, with their more complex minds and greater manifestation of pure logic, can instantly detect all spoken or written lies. A modron cannot be magically compelled to act contrary to its nature or its commander's orders by any means.   Modron squadrons have the power to cross planes easily and at will. When they do so, they manifest weapons made of the same damage-impervious metal as themselves. (Modrons do need to defend themselves, as they are capable of being hurt and killed, even though their metallic bodies never show any signs of damage. Any damage to modrons seems to occur exclusively to their metaphysical energy.) The type of weapon they are able to wield corresponds to their body plan. They can sometimes lose these weapons; left-behind modron weapons are incredibly valuable to mortal sages and warriors.