Nether is a dark, cold substance of unknown origin, often theorized to be foreign to our world. It has been found to naturally consume light and heat, and in its natural form is destructive to nearly all types of matter, rather like a powerful acid. However, it has a reaction to ferrous materials (such as iron) that causes it to disperse and dissolve. It is also very susceptible to thaumaturgic light, which burns it away extremely effectively. Fire and other sources of extreme heat are also effective at burning away nether.
The origin source of nether is unknown, though theories abound. Some argue that it is a natural part of our world and is all around us just waiting to be concentrated into physical form, while others claim it has otherworldly origins - perhaps some aspect of the world of the dead that occasionally seeps into our own. Whatever the case, the presence of nether is rarely observed outside Tol Rhun, the Lost Lands, and the Raffaella region of Ivial.
Many researchers describe nether as "pseudo-sentient", saying that it is capable of acting on its own. In large enough concentrations, it can even condense into nethrim creatures known as phantoms that appear to have at least some measure of will. This phenomenon appears to be more common in areas with greater concentrations of nether, and is perhaps most frequently observed in areas associated with death, such as graveyards or crypts. Because of this, nether is often seen as some sort of dark or malevolent force, and is widely believed to be able to latch onto or "corrupt" souls. Additionally, nether has been known to take root in both plants and animals, and then spread as what it has latched onto transforms into a nether-based imitation of its original form. All this taken into account, it is perhaps understandable that most people are mistrustful and fearful of nether, and would go to lengths to purge or otherwise remove it whenever possible.
Despite the widespread mistrust of nether, there are those who would learn more about it, looking into both its nature and its potential applications. The sorcerers of Tol Rhun have embraced nether as a tool that can be used to to achieve great things, their nation being one of the few places where sorcery can be researched without suspicion or stigma. Additionally, Rook Parlour is rumored to study the properties and applications of nether as they delve into the secrets behind life, death, and the soul.