There are some schools of thought that say the abyss and the sun are the same wild energies that created the big bang and thus compliment each other rather than “cancel out”. These philosophies are long debated, no one is really certain on the preciseness of how it works.
A long, long time ago, in the time long before the prophecy, the Ten Realms were perfectly balanced. In the midst of a catastrophic event, the doors closed between all the realms and the Aether that carries all the energies of the elements became stagnant, festering as they were. Naturally, the Abyss did what it had always done; without the Sun to balance it, it began to corrupt all that it touched. It mutated species, flora and fauna as well as changing the climate and environment itself. Deep within the soils of the worlds closest to it.
But as time passed, as the prophecy came with the Black Rose Eclipse, the veil began to thin and the Aether began to flow again through the cracks through all the worlds. But time was not kind… for now instead of perfect balance, the abyss flows into worlds like an oil spill on the ocean. All of what it touches is corrupted, changed or eaten.
At first it was hardly noticed, but since the prophecy the weakness of the veil has become more known. Those who watch the veil, like the Order of the Black Rose, are wary of the cracks and doorways now; doing what must be done to close them and prevent the spread as much as possible. Despite every effort, every realm now has abyssal corruption growing within it, even if it's in the deep undergrounds.
If one comes into contact with the Abyssal infection, it is hard to say what will happen for the effect never seems to occur in the same way twice. Though, across the board it shares common traits with abyssal spell casting such that it eats at life or mutates it.
The newly abyssal infected entities will show it in deep black veins that will slowly feather out until flesh is pure in darkness. They will shy away from the light and seek life to consume. For once one is touched by the abyss, it seeks a price to live and that cost is the life of others, to spread, to learn, to devour. In some lesser cases it can be treated by using Sol Magic of the light/sun that seems to burn it out.
Those born of the Abyss or abyssal aligned already pay the price in some way, such treatments cannot cure them. But they themselves would never know they are sick with the abyss anyways. Those who use frequent abyssal based spell casting may already be tainted, and if not yet, they will be if they lean too much to the darker side.
The Thinning of the Veil
The esoteric science for opening new doorways has been long lost since the “time before” and pathfinders have been sought out and killed by those who wish to prevent the prophecy. Despite all of their efforts, the Aether seeks to rekindle its path and flow.
It is in areas where Magic of opposing elements occur, that a weakness in the veil is created, though it is not the only way for this to happen. Strange experiments, those who Aether or shadow walk… covens utilizing too much energy or the use of powerful enchanted artifacts - These all slowly chip at the veil in the locations they are done in. With it, Aether energy begins to seep through the cracks… these wild doorways can be tiny or large depending on how long they have been left to fester.
Where doorways have been open, are open or where cracks remain in weak points of the veil, pure wild Aether energy slowly settles to the ground, crystallizing in veins beneath the soil, inside rocks and caverns. It can contaminate water springs that have curious effects, such as the lore for the "Fountain of Youth" and "The Holy Grail".
These crystals illuminate in deep blue hues and grow no more than 3-4 inches long. They are rarer than gold to find but their value to a Magic wielder is priceless. They are the physical manifestation of magic itself and thus extremely volatile to mine or work with. They are as temperamental as explosives and unpredictable in effect.
Despite the dangers, those rare few who know about the crystals will linger around doorways or hunt for thin areas of the veil to harvest it at the potential cost of their lives. These are far and few between, for as rare as it is, these crystals seem to only pop up as anomalies in private collections, or on the shelves of alchemists in its more liquid refined serum form.
The last civilization of scientists that ever took the time to study it, were the humans of Atlantis, who foolishly built their city under the waterfalls that fell from the sky (a doorway to Pelagius). They used the crystals to power their cities and technologies, which were more advanced than the modern era. They made astounding progress in the research of the crystals, but all was lost when instabilities occurred. The heavy use of the crystals became volatile and interacted with the doorway… This resulted in a huge explosion such that the city of Atlantis was consumed, and doorway shut. Though no one truly knows this, for as the legend says, 'Atlantis sunk into the sea.' All research and knowledge of Atlantis was lost, and the Human world was set back thousands of years in esoteric knowledge. It is theorized that knowledge of how to use and refine these crystals would be found if only Atlantis itself could be found.
These crystals exist in all the realms, however some have never seen them as they become explosively consumed in volcanic activity. Places such as Talamh have never seen them as the Fairy Folk harvest them to make fairy-dust.