The Shadowfell, Demiplane of the Forgotten, exists as the void left by the Feywild. If the Feywild is a plane where time stands still, the Shadowfell is a plane where time does not exist. Where the Feywild is bright and vibrant, the Shadowfell is black and inky, devoid of color. The recreation of a blind Goddess, The Shadowfell is Thecla attempting to fill the void with her memories of the Material Plane and Feywild, thus it is an amalgamation, as well as a corruption, of both places.
The Shadowfell is where things go when they are forgotten, long dead kingdoms still stand tall over the inky wastes where they once stood, and those who died without a heaven or hell to go to end up in the Shadowfell. While the Feywild is a plane of infinite and ever-present time, where seasons overlap and day and night happen side by side, the Shadowfell is the dark contrast to it. Time does not happen in the Shadowfell, it is neither day nor night, neither winter, nor spring, nor summer, nor autumn. Time stands still in the Shadowfell, and this can take a toll on a mortal body and mind.
The Shadowfell gives off a eerie sense of familiarity, but also a strange sense of novelty. What is now a grand city in the material plane, may be a small hamlet in the Shadowfell, or might not be there at all. Sometimes, if enough beings from a particular place find their way to the Shadowfell, they can exert a level of control over the Shadowfell, placing that location inside the plane, or correcting details that went mistaken. Similarly, if a location was forgotten about by the world, perhaps, destroyed by a great disaster or war, it may end up in the Shadowfell, where all forgotten and discarded things end up.
Some liken the Shadowfell to the dream of the Goddess Fenjja. While most intact minds can exert a level of control over their surroundings, and multiple minds exerting the same control compounds on one another, Thecla is omnipotent and omnipresent in the plane, and can destroy anything that another being creates if she wills it. Thecla is blind and in everlasting pain, this is reflected in the Shadowfell. The Shadowfell is devoid of color and light, yet all with vision can still see in the plane, which can lead some to madness due to being unable to comprehend the nature of the plane.
Additionally, dark beasts lurk and appear in the Shadowfell from time to time, as Thecla’s pain manifests. She is unable to control the manifestations, and although she is benevolent to those in her domain so long as they are in kind, she cannot prevent harm from coming to them from these manifestations. The dark beasts, as well as the lack of color, light, and time, make the Shadowfell as inhospitable from any evil plane. Still Thecla cares for all those she has taken in, and does her best to prevent them from being harmed.