The Dark Aether is tied to the Material Plane, Plane of Aether, and the Planes of the Dead. It separates the living from the dead, where as the Plane of Aether is the very essence of life and growth the Dark Aether is the opposite. Eternally shrouded in mist, darkness, and cold it roughly takes on the shape of the Material Plane, though as if the material world were blasted with deep bone-chilling cold and darkness. There is virtually no natural ambient light in this plane except from the mist and ice which crust over every possible surface and fill every space of air.
Inhabitants
The Dark Aether is where those forced out of their normal existences most often go. Filled with demonic entities of all kinds, trapped souls banished from their bodies, and all manner of dark creatures intent on feeding off the living. Shortly after the Peace of Ages on Elder Soteria, the plane was suddenly filled with light and dark fae alike. As such there are all manner of creatures, though most have gone insane over the eons trapped within the Dark Aether.
Fog of Memory
Those newly trapped within the Dark Aether will have difficulty remembering their lives outside of the Dark Aether. Although over time their memories will return, it is often tainted somehow once happy memories become corrupted in various ways, sapping away ones will to return. This is not perfect and with effort ones memories can be retained and brought into focus - once brought into focus the memories solidify and remain protected from corruption thereafter.
Survival
A living being in the Dark Aether must still survive as in eat, drink, sleep, and other such things. This is difficult, though plants and animals have shadows within the Dark Aether that come in and out of the endless mist. They are representations of the actual creatures, though within the Dark Aether they can be killed and consumed, it lacks a pivotal quality of sustenance. One can survive from consuming them, but barely. If a living being kills another living being also in the Dark Aether, the consumption is the way it would be within the Material Plane. This leads many to seek out living beings trapped within the plane to consume as the shadows never quite quench ones thirst or hunger.
Sealed
The plane is sealed off with powerful runes echoed in the Material Plane as well as the Plane of Aether and the Planes of the Dead. The origin of the runes is unknown, though it is possible they came to be when the plans to banish the Dark Fae from Elder Soteria were executed. These powerful runes further divide the living and the dead, the Dark Aether sealed against either. It is also possible that the runes were always there, created in the primordial origins of the universe by hands unseen.
Entrance / Escape
The seals are not perfect, though it is clear they are supposed to be. Within the plane there are Druthonae, connected to various times and places and as such it can also be reached via the Verum Transitus. Transition is very difficult, requiring a living being (mortal or not) to be protected by powerful magic or they could become trapped there as soon they enter. The plane itself resists escape, changing the path to Druthonae and hiding alcoves where the Transitus can be used. Those who become trapped often forget why they are there or how to get back, even if a doorway / portal exists directly in front of them they often simply wander off.
Eternity
Within the Dark Aether there is nothing but eternity, no entity may die in the Dark Aether. Since the plane is sealed from others, anyone who dies within it simply stays there losing a bit of themselves. Mortal beings trapped within lose their minds a little more each time they 'die' until finally, there is nothing of them left. Time has very little meaning, it is both endless and stagnant. Thoughts can send someone back in time or forward in time, so long as it is within the Dark Aether.