The Shadowfell

The Shadowfell is an approximate mirror image of the Material Plane. The biggest difference is the sky, which is completely starved of celestial bodies, and is pitch black for most of the day. The earth is illuminated by a uniform, artificial light; unlike the calming glow created by the Mass Resurrection spell, it appears fake or even harsh to some people.
During what passes for night in the Shadowfell, the sky is slightly colorful and the earth is black. It's like midnight on the Material Plane, though the color of the sky has a wider repertoire. Some places in the Shadowfell have much stranger visual effects, signaling the presence of creatures with a close attachment to the plane.
A few deities are active on this plane, but the most influential forces are the atmospheric shadows, embodied by the god Oscuridell. These forces are constantly adding and modifying things, but most people are safe from them now. The Shadowfell comes in various levels of wilderness: the parts closest to the Material Plane are stable, comfortable places to live, while other zones are so feral that all but the bravest creatures find them deeply upsetting. Most people on this plane stay far from the capricious shadows. But some people are still in danger of encountering, or becoming, an assortment of crudely sculpted beasts.

Civilization

Huge cities are built with only the materials offered by the Shadowfell. Others are constructed with help from adjacent planes. Because of this, society on this plane comes in various forms: corporeal or shadowy, sensible or absurd. All in all, there are hundreds of thousands of people living in the Shadowfell, and possibly many more off the grid.

Gateside Towns

In some towns, the buildings and roads are based on material stuff, "real" stone and lumber imported through the nearest black gate. This keeps them stable in the ever-changing Shadowfell; it takes a lot of work to maintain, but at least the residents don't have to change their maps every year. Each building's steady foundation prevents it from wandering off, but you can still see the shadowy terrain flowing in the spaces between, as if the town were built inside a slow-moving river.

Shadow Towns

The shadows can also copy manmade structures. If a building on the Material Plane stands for 100 years (or shorter, in some cases), a parallel one builds itself in the Shadowfell. Because of this, even in the untouched wilds there are empty houses and monuments resembling Material villages. These "shadow towns" are sometimes completely empty, and explorers love to seek them out, at least for the novelty of spending the night in a decadent mansion.
A shadow town makes a good foundation for an actual town, since it already has some housing and infrastructure. They might be folded into an existing civilization, which implements farms and buildings in between the shadow buildings. An isolated town could also be claimed by eminent creatures, who twist the location to suit their needs and populate it with ghostly figments. Visitors to these towns may think they're empty, but they should make sure it isn't haunted before they trespass.

The deep wilderness

The Shadowfell used to be a subject of primal fear. It may not be anymore, but its frightful elements are still around, shooed and stowed away in the hidden corners of the plane. The ground is infamous for buckling under people's feet: anyone who stays there begins to feel like there's a world of horrors just beneath them, and they can't trust a footstep to meet solid ground. The sea floor is even worse.
Development in the Shadowfell slowed almost to a halt when the Duumvirate War started. Some parts of the Shadowfell are no longer controlled or observed, and the inhabitants have retreated to focus on the war. The people of the Shadowfell are afraid to find out what's been left to germinate there.

Geography

The materials composing this plane are shadowy, dynamic, even alive in a sense. Its geography is similar to the Material Plane because it actively tries to be. Over the years, it flows into the shape of the landmasses it wants to mimic, reversing changes to itself and copying changes to the Material Plane.
When it isn't copying the Material Plane, the earth breathes and shifts over time. This may technically be true of the geography of any plane, but the Shadowfell in particular is a troubled sleeper. It tosses and turns, assuming a markedly new shape every year.

Imperfect Copies

Sometimes the copies of the Material Plane are flawed. Some buildings are bent and on the verge of collapse, stuck in an equilibrium between falling over and rebuilding themselves, breathing out mist as they try to stand upright. Likewise, ancient trees may grow twisted and buckled, and sandbanks produce clouds of fog as they flow toward the sea and pull themselves up again.
These imperfections can also lead to deep faults in the ground, opening into a massive underground world beneath the Shadowfell. The tunnels shift, split, and reconnect themselves over the years, like air pockets in solid earth. These underpasses could be used to move securely across the plane, but it's hard to tell what else is lurking there, since many of the tunnels seem to have no end.

Extreme Locations

In places with dense shadows, the plane can get a little creative. Three-story buildings could appear as three hundred stories, with the same floors copied over and over. Other buildings are sculpted in a way that would never happen naturally. Some changes are exaggerations of natural forms, from quicksand and whirlpools to impossibly balanced structures which are completely out of scale. Creatures with shadow nature can make their own adjustments to the plane's geography.

The Sea

On this plane, travel by sea is close to impossible. The water is opaque, usually pitch black, and people above the surface can't see what's underneath. Even if they scan the sea using magic, not much can be done to prevent a boat from being sunk. And when someone falls into the water, the disruption can be sensed by sea-things for miles around. It's possible to fly across the sea, with enough endurance, but it's easy to get turned around: there are almost no landmarks, no way to gain a sense of direction, and the churn of water and fog is hypnotic.

Fauna & Flora

Though its light is mostly invisible, the Shadowfell has a sun, and plants grow even in apparent darkness. There are also shadowy copies of plants from the Material Plane: century-old trees have perfect reflections in the Shadowfell, and grasslands are imitated by a ragged carpet of shadowstuff. Flora from other planes sometimes take root in the Shadowfell by traveling through purple gates. In particular, Deisos is a country burgeoning with gardens, rich in both material and shadow flora.
Likewise, the animals of the Shadowfell are usually carried over from the Material Plane, though they're liable to warp under Shadowfell Influence. In the wilder parts of the plane, those spaces between civilizations which defy attempts to settle or map them, a promenade of absurd nightmares and cruel caricatures can be found. Falsification, as rare as it is, has transformed many a lost animal over the years. Those afflicted have an extremely long lifespan, and their numbers have been increasing slowly but surely. They're usually content to wander the plane, silent, alone, and sleepless. It's a very bad sign when fell creatures decide to flock together.

Natural Resources

The most important resource offered by the Shadowfell is landmass: the people there have plenty of room for crafting, farming, and raising families. It's also a covert way to travel across the continent, though it's hard to navigate without a visible sun. The Shadowfell's physical resources are scarce, since its shadows dissolve in other planes, but some magicians can make something of it.

Type

Dimensional plane