No one really knows entirely what exactly the shadow realm is. The discovery of the shadow realm is a very recent one, although its effects have been known for at least 30 years. The corruption that scars certain regions of Celenia seem to affect both flora and fauna and it's believed to be a direct effect of the shadow realm upon the natural world.
What is the shadow realm?
As for what the shadow realm truly is there are certainly speculations. Hard facts are hard to come by as the only information available is thousands of years old and written in a language long since dead, or ramblings of fanatics and cultists. What is known is that it's some form of parallell universe or metaphysical plane of existance, wholly different to the Dunian universe. One can pass through the Veil to the other realm and exist there fully. The two realms seem to partially correlate space and time, but not fully. Time passed in one world isn't necessarily the same or even relative to the other, nor is space. One can move in one world and come out near or similar to where one would end up if one moved in the other world, or one can end up days or even weeks of travel away.
There is no one that have figured out how these two worlds relate to one another or managed to have made any observations that points to a repeatable experiment to correlate time and space between the two realms.
Crossing the Veil
Furthermore passage between the two worlds is a very dangerous and awkward endeavor and there is no documented way of accomplishing this without using one of the artifact Mist Portals, which in turn require some form of power source to run and can only run for mere moments at a time. Those power crystals are extremely hard to come by and no one knows how to manufacture new ones, or even recharge the old ones.
Passing without a portal
There are reports of people managing to cross the Veil with sheer force of will, but it's unclear how this has happened. There are no documented scientific tests of this and quite frankly, no one even knows where to start. The rumors about people crossing over is considered to be lunatic talk and have been encountered mostly in cultist circles.
A breach in the Veil manifests as an area of blurred reality. It's not simply a rift or an opening, but the two worlds seem to almost occupy the same space at the same time. Both worlds existing in a strange, ghostlike state within the rift. These locations are usually called "Shadow Scars" or "Mist Portals", although the latter is usually reserved for constructed portals that allow passage through the Veil. The name "mist portal" comes from the purple, swirling, mistlike substance that permeates the air within these portals or scars.
These breaches offer the most efficient way of passing the Veil with using excessive amounts of energy to run a portal. The only problem is that the breaches seem to be a natural phenomenon only occuring within the shadow scars of Celenia and at random. This makes them not so reliable.
The Horror beyond the Veil
There is a another problem with passing the veil. Creatures from the natural realm do not fare well in the shadow realm. There is something about it that causes a sickness in everyone and everything within less than a day, leading to a horrifying death by suffocation. It has been called Shadow Sickness.
The Shadow Realm itself is a dark and desolate world. It's skies are pitch black with dark, purple auroras dancing across the skies. The world itself is almost devoid of any plant life, only shrublife, dead or dying trees and thick, thorny vines seem to live in this hostile environment.
The surface is shattered and cracked, frequently falling off into endless voids. It's as if the entire world hangs in an endless void, with cracked and free floating landmasses. The surface is mostly rock, covered by a thick layer of dust or a sandlike substance.
The entire world is draped in a purple darkness and it's frequently covered in a thick fog, making traversing the world an extremely dangerous prospect, considering all the endless pits.
Also known as the Black Death, shadow sickness is a fatal toxic reaction to the energies of the Shadow Realm.
Native to the Shadow Realm, this hive-minded species is made up of a multitude of biological castes, each designed to fulfill one role or another. They are highly aggressive towards invaders of their realm and lethal in combat.