The Invisible

The Invisible is a world on the border of Aedrelon. It exists parralel to it, sometimes getting really close to reality but never exactly reaching it. It is the home of creatures and ideas that don't have a place in the waking world, the home of ghosts and curses. Some creatures across Aedrelon are able to perceive the Invisible, and briefly gaze upon its strange inhabitants, but just as the Invisible can never touch Aedrelon, beings of the waking world can never really comprehend what lays on th other side.

The landscapes of the Invisible

When watching the Invisible from the waking world, it appears as an ethereal world in fiery blue hues overprinted on reality. The Aer in everything blurs the edges of the world and seems to dance as a fire. Around all that exist, the sparks of Aer take on some different colors, shapes and brightness, from deep purples to pastel reds or soft yellows, to reflect that entity's unique Arcane Signature.
It creates a vibrant world that is difficult to precisely discern, but sufficient practive allows one to read the Arcane Signatures around them to intuite their surroundings with extreme accuracy, sometimes even more so than if they were using their eyes. Some blind or visually impaired people have even been reported to lead perfectly normal lives simply by focusing all of their senses on the Invisible and the unique flow of Aer they perceive there.   No structure unique to the Invisible can be observed there. The Invisible world itself is identical to the "normal" Aedrelon, with Curses and Ghosts being the only sights that do not exist in the waking world.

Curses

Curses are beings uniquely found in the Invisible. While most of them usually take the form of small insect-like creatures, there is absolutely no limit to what a Curse can look like. They are living things born from Aer just like everything in the waking world, and are thus victim to its whims all the same. Some are akin to spiders, to crickets or flies, other are closer to insectoid rodents, while some rare ones could be absolutely indistinguisable from an Epreas, a dog or a tree. The only thing that differentiates them from the rest of existence, when seen from the waking world, is their pitch black Arcane Signature : a low, rumbling black fire that spreads to everythingthe curse touches.
Indeed, another quirk of Curses is that they are able, from the Invisible, to latch onto things from Aedrelon to feed off of their Aer. By doing so, the host is almost always inflicted with some sort of strange disease. There is no limit to the symptoms caused by these diseases : a simple fever, an irrational fear of water, an unquenchable thirst for blood, the sudden transformation of the body into tree bark, many more and an almost infinte amount that hasn't been recorded yet. While some of the most benign symptoms can be aleviated by traditional medecine, the only way to make them disappear or to prevent them from worsening is to rid the victim of the Curse. To do so, requires the help of an Exorcist.

Ghosts

Ghosts are, just like Curses, beings that only exist within the Invisible. They are the manifestation of a mortal being's Aer lingering after they have passed away. This mass of lost Aer maintains a form similar to the one the being had when they died but is absolutely devoid of will and can no longer act as it used to. Most Ghosts keep a few core memories from their mortal lives and tirelessly repeat some of their old habits, as if to convince themselves that they are still alive. The cases of actively sentient Ghosts, capable of the same nuances in thoughts, actions and emotions as when they were alive are extremely rare.
In the same vein as Curses, Ghosts can be exorcized if they happen to pose a threat to the people of the waking world. While most Ghosts are harmless and couldn't even cause harm if they wanted to, those who died particularly gruesome deaths or lingered forgotten in the Invisible for too long sometimes turn into a Malevolence.   Malevolences are Curse-like Ghosts, able to subtly act on the waking world. While they can't latch onto someone and absorb their Aer like Curses do, Malevolences can spread their feelings and desires to living creatures and have them act as the Malevolence would. Cases have been reported of entire town populations hunting the murderer of an old lady for years to get revenge, after the old lady's Ghost had turned into a Malevolence.
People affected by a Malevolence are still conscious and effectively "themselves" but are compelled to act upon the wishes of the Invisible force.