The Grind of Acheron

There is a realm where the obdurate, crystalline structure of a Mechanus shatters and fragments, floating free into the void of half-formed concept. This is a border, though not any physical border because the clockwork nirvana is infinite and redundant in its mechanism, but a conceptual border where the Prime Mover's certainty no longer holds, where the grand program fails. This was the place where, after the fall from Unity (as Law sees it), rebellious Archons sundered themselves from Mechanus. This is the ideaspace separating Pure Law from Hell. This is Acheron.

It's a hell of sorts in its own right. Its acolytes know it as the Crucible. Here, they contend, new truths of Law are formed. Perhaps one day there will be one stronger and surer than either Hell or Mechanus? Adepts of Pure Law view it as gall on the purity of Order, the place where Hell's error abrades it. The Lords of Hell see it as an opportunity.

Pieces of supernal machinery break off at the edges of Mechanus, twisting and reforming, to store failing Order within, into planet-size Platonic solids which continue to degrade, erode and crumble. These have been colonized by numerous beings: malcontents from Hell, reformed things of Chaos, and authoritarian souls with iron dreams of their own version of Order. All the would-be dictators and tyrants begin to gather their followers among the lost and the beaten and forge their own armies of conquest. And then they go to war.

The struggle is as senseless as it is endless. None of the despots or authorities are ever able to overwhelm the others and seldom do they convert them. The strength of Law is shattered, after all. Also, none of them have clear vision of Unity, for they were only born after it. They merely ape what they know of Hell, crudely. 

One might be tempted to view Acheron as a place of Chaos, but philosophers point out that when taken as a whole, the plane is as predictable as Mechanus. Its war machines grind forever on at the behest of devils who will never achieve the godhood they crave.