The Shadowfell is fundamentally a realm defined by darkness. It is not inherently evil, but it serves as the origin and dwelling place for malicious emotions and thoughts. Every negative thought and feeling fuels the Shadowfell; its existence depends entirely on this energy. Without it, the realm would gradually collapse and cease to be.
Fortunately for its continued existence, an endless supply of such emotions flows in from the Material Plane. These emotions and thoughts are the very building blocks, shaping the terrain and the creatures that inhabit the area. This emotional energy flows through the landscape like active rivers, dramatically altering extensive areas as they travel.
The realm operates on a 1:1 scale with the Material Plane, meaning that the negative energy is more concentrated and intense in areas corresponding to large cities in the Material Plane.
If one of these energetic rivers encounters a living creature, the creature absorbs the energy, causing it to be transformed into a physical manifestation of the thoughts and emotions it took in. This mechanism naturally leads to creatures growing more powerful the longer they survive within the realm. Otherwise, the river’s slow influence on the environment results in rugged, harsh landscapes: towering mountains and deep chasms surge throughout the Shadowfell, making travel perilous. The thoughts and emotions are literally ingrained into the earth itself.
Unfortunately, the danger does not end there. The realm is also home to vast, empty, dark swamps shrouded in thick, zero-visibility mist, where creatures lurk, waiting for a visitor to make a single misstep. There are also lakes composed of a rough, fast-moving liquid that will strip the skin from any creature that touches it.
Creatures are often spontaneously formed during moments of intense negative energy, resulting in beings whose strength is directly proportional to the amount of negative energy that catalyzed their creation. The specific form these creatures take appears to be contingent upon the negative thoughts that brought them into being. Consequently, the Shadowfell is quite literally the physical embodiment of nightmares.
The most notable inhabitants of the Shadowfell are the Dark Elves, known as Shadar-kai. They are the most unique creatures residing here; while they are initially formed in the same manner as other denizens, they also possess the ability to procreate. This means their continued existence is not reliant on the continued health of the Shadowfell.
They exhibit seemingly superior magical abilities compared to the average human, but they are physically weaker. This tendency leads most Dark Elves to become magic casters. They have adapted to the severe environment by constructing massive structures atop mountains for their homes, which has naturally granted them an increased resistance to oxygen deprivation. They are known to follow the commands of the Raven Queen, though it is not clear if this allegiance includes the entire species or only a select subset.
The Shadowfell is not a distinct, entirely isolated realm as some might assume. It lies just beneath the surface of the Material Plane, accessible to any magic user skilled in plane shifting. Furthermore, any creature with enough pent-up negative emotion inside them can, under the right circumstances, force their own entry into the Shadowfell using that very emotion. This action creates a short-term tear in the fabric between the realms, lasting anywhere from minutes to days. However, the creature who caused the tear becomes merged with the Shadowfell; they are reborn in their negative emotions, reformed and recreated into an entirely new entity. Creatures made in this manner are the only sentient beings in the Shadowfell other than the Dark Elves. Occasionally, creatures thus remade will attempt to return to the Material Plane through the tear, unleashing chaos upon those who were the source of their despair.
However, like all creatures born in the Shadowfell (with the sole exception of the Dark Elves), they decay when on the Material Plane. Positive emotions and hopeful thoughts act as a poison to a Shadowfell creature, causing them to slowly crumble into dust. Dark Elves are the only known Shadowfell creature that does not suffer this decay.