Aether

Æter (also written as aeter or aether) is a building block of the Dark. Its exact description is hard to create, because of its elusive and everchanging nature. It is simultaneously liquid, solid, gas and - to a point - something similar to rays or light. It can succesfully mimic stone, metal, air... or living creatures, including every single part of their body.
In short, æter is a magic in its purest form. Daemons, gods and afterlifes are all created from it. It also exists in the Light, but mostly in its immaterial, 'radiation' form, sometimes called ambient magic. It can be found in the atmosphere in relatively low ammounts, also beneath the ground where they tend to form veins, where it is however bound to matter.
Mortal races do not possess an ability to use aether in its 'radiation' form, as they require in it either gas, solid or liquid form. The best source is killng daemons - boh those materialized from aether and those physically manifested contain aether in their bodies. It can be then extracted and used.

Aquiring

Aether is aquired in the place where daemons are most abundant, mainly in the magical anomalies, where Dark is nearly fused with Light, and the barier between them is practically non-existent.
In its pure form it is however extremely toxic, capable of causing mutations in living creatures, if swallowed or touched. It is possible to - if one is cautious nad careful - safely aquire it and store it in specially enchanted bottles, that doesn't allow it to vapourize. After few days the so called natural æter changes into stable æter, attuned with the Light to the point that it is no longer capable of simply disappearing. There still exist a problem of toxicity and mutations.
Stable aether is then processed into purified æter, known also as refined and pure. It is done in well guarded aether refineries. The effects is safe and non-toxic version... and a lot of ultratoxic wastes called black aether, Imperium of Karadia tends to utilize it by throwing it back into Dark. Which is safe way of making sure noone gets killed or mutated.
Purified aether can be described as intensively blue water, slightly heavier than normal. It can be drinked which helps mages replenish magic energy. It can be also used to restore power to depleted enchantments, variety of daemon summoning rituals, and in a hundred of different ways.

Aether Poisoning

Aether poisoning is a thankfully rare problem, that however occus from time to time. It happens to several type of people. Most of them are refinery workers that forgot about safety procedures. Second are mages experimenters, especially those dealing with genmaturgy and using black aether to cause mutation in plants and animals. Third are victims of magical disasters, like for example a refinery explosion, short-time and uncontrolled opening a hole to the Dark due to for example failed summoning. Fourth are people frequenting areas that were corrupted with magic (for example magical anomalies), that however tend to be only lightly poisoned, because they developed resistance to it over time.
Light poisoning symptoms are similar to possesion. Headaches, auditory and visual halucinations, paranoia, fits of madness, narcolepsis, convulsions and sometimes aphasia. In case of stronger poisoning, the symptoms end up... divided.
70% of victims end up mutating, quickly changing into something that is less and less similar to what it once were. Most end up as still conscious stains of blood and flesh, that are finally mercy-killed.
25% of victims also mutates, but not in ways that are purely detrimental. They are of course completely random and oftenly carry side effects, although do not need to be 'treated' with euthanasia. The severity of changes depend on how strong the poisoning was. In light cases it ends with changs of eye or hair color. In stronger the situation is... much more severe. Nightvision. Being able to see through illusions. Muscle strenghtening. Growing up a set of horns. Capable of understanding unknown languages by rudimentary form of telepathy that allows to understand the meaning behind the statements. Such mutations are nearly always coded by recessive genes, making them very hard to inherit even in the case when they weren't accompanied by sterility.
Remining 5% of victims can be summed up by 'others'. It is a combination of all possible effects of aether poisoning that does not fit the mutation schematics. Including spontanoues ascendence, changing into energetic 'vampire' that siphons magic from air and living creatures, losing physical fom and becoming a shapeless and bodyless intelect, gaining ability to use magic without contracting a daemon... but also spontanous combustion, brain death, extreme forms of several mental disorders, not to mention few confirmed case of spontanous gender or specie change. Or turning liquid/gaseous. There was one case of a person changing into a tree.