Doommarch
Pockets of Doommarch, or simply the Doommarch, are the phenomenon of randomly appearing and disappearing "pocket planes" on ../PlaneOfExis and ../PlaneOfSepulture. The terrain and climates of pockets of Doommarch are almost always inhospitable, and they may depart as quickly as they have entered, or they may remain "on" the plane permanently, replacing whatever existed before it.
Properties
Very little about Doommarch is known to residents of Exis. Their chaotic nature of appearing on the plane, combined with general reluctance to discover the methods of planar travel necessary, have left scholars with little opportunity or desire to understand its/their nature, despite the destruction caused by the arrival of Doommarch.
An important property as of yet unknown is whether or not Doommarch is a plane, a collection of smaller sub-planes, or a number of unrelated planes. Limited evidence, such as the disparity among instances of Doommarch, would point to it either being a classification of planes, or some form of planar entropy which causes unrelated planes to manifest. There is no concrete evidence pointing towards any definitive answer, however.
It is known that when Doommarch manifests, it replaces the existing terrain outright, as if the location of two pieces of land had been swapped. The fate of the replaced plane is not guaranteed; some have returned entirely unmolested, others ravaged and pillaged or teeming with bizarre creatures, and others still doomed forever, with the Doommarch never receding. Life can survive the jarring process, but most creatures — if they still live — return to their home world shaken for years to come.
Geography
The planes themselves are generally barren wastelands, teeming with bizarre creatures and mad forms of life seen on Exis. These denizens — if they truly inhabit the plane and are not merely captives of it — are almost always hostile towards Exis natives (although they are curiously benign towards Sepulture natives) and relish the opportunity to escape to the plane of Exis, being the only known form of travel between the two planes.
In general, Doommarch is composed of barren rock, storm clouds, and blistering heat. Many exceptions have been noted, however, with Doommarch instead consisting of great crystal palaces, or houses of giants, or dark voids which no light permeates. No known maps, or even attempts to map the landscape, exist, and no attempt has been made to map manifesting and receding Doommarch to determine the overall nature of the phenomenon.