This is part of the update to the modrons that I'm working on. It's meant to explain recent events involving the modrons and the formians from a point of view that acknowledges the existence of the modrons (as opposed to WotC's PoV). Right now, of course, I'm just discussing the slide of Menausus and its effects on the formians, the modrons come into this in the next sections.
Menausus
(a planar sage speaks on the slide and its effects on the layer's number one inhabitants: the formians)
I think that by now every planar knows of the slide of Menausus from Arcadia to Mechanus. The factors of the Harmonium tried to keep the news of their big mistake quite, but a whole planar layer doesn't just go missing without people noticing within a day or two. That the Hardheads managed to do so for several weeks is a tribute to the appropriateness of their common name. Eventually, however, the full scope of the disaster became known, and the slide obtained the dubious honor of becoming the most studied event in recent planar history. In a period of weeks, literally millions of gold pieces were spent on ever more sophisticated divination spells, their magical components, and their research, until every aspect of the disaster had been recored and analyzed hundreds of times over.
Final Thread
Ironically, though the disaster will go down in history as the Harmonium's biggest blunder, the proximate cause of the slide is one of the few details of the disaster that remain dark. Of the reasons for this, there are two major theories. The first, and most widely held, is that the final trigger was so minor and commonplace as to be completely unnoticeable, the straw that broke the camel's back, if you will. The other is that the slide was set off deliberately by an individual or a conspiracy powerfully warded against divination spells, and so undetectable by our retroactive scrutiny.
Sea Change
Even before the layer came to rest in Mechanus, fundamental paradigm shifts were taking place in the formian hive consciousness. The cause was the severance of the layer's connection to Arcadia and its growing connection to Mechanus, which was reflected in the formians' outsider souls. These changes further shifted the alignment of the layer, which meant that within minutes of its arrival in Mechanus, the layer was more anchored there than it had been to Arcadia since before the Harmonium first set up their camps. Back-time scrys shows the formians completely inactive for a further half-hour after the slide, with some pick up among the higher castes toward the end of this period.
Once the formians were awake, however, they moved fast. Within forty minutes of the slide's completion, worker formians began to arrive at the Harmonium camps. The camp commanders were informed that their operations were infringing on formian territory, and were given a choice. They could chose to evacuate their camp, or they could demobilize their guard force and open their gates to the formian taskmasters. They were further informed that failure to complete evacuations or signal acceptance of formian rule within the time limit would result in immediate attack by overwhelming force. The time limits were harsh, but based on a honest assessment of how long it would take to evacuate the camp in question.
Harmonium Reaction
The Harmonium may pride itself on its ability to react quickly and sensibly in the face of danger, but they failed themselves in this instance. Word of the formian ultimatums reached the factors before the full scope of the situation was understood. Consequently, much time expired while the factors dealt with contradictory reports and evaluated plans of action. In the end, the smaller camps were ordered to evacuate back to Sigil and Buxenus, while the larger camps were reinforced with Harmonium elite troops. This operation was complicated by the belated discovery that most of the Buxenus portals had been severed by the slide.
Consequently, the Harmonium redeployment was only partially completed when the formian ultimatums expired. As promised, each camp was hit by a large force of formian warriors, sometimes with contributions from more than one hive. Camps which survived the first attack found themselves attacked again within hours, and with ever escalating force as more and more hives contributed warriors to the effort.
Growth to Capacity
Over the next twenty years, the formians seeded new hives onto every cog in the Menausus zone, expanding their population to the maximum allowed by their territory. Once this population boom was complete, there was a further ten year period of consolidation, during which the balance of the formian population shifted towards warriors and armadons rather than workers. At the end of this time, in perfect response to their internal population pressures, the formians launched outward, into Mechanus proper.
Incursion Zone
The Counter Thrust
personally, I've always liked the thought the annexation of the layer from arcadia to be a victory for the modrons. The multiverse has turned significantly towards law. That makes the modrons a force to be feared.