It is not known to any except Morrow's Shield and a few trusted retainers, but in the waning days of the Infernal Insurgency the Demon Lord Xixivis pulled something from Outerworld into Sarella. That item was a large blue crystal, the seed of Arcanium that would threaten to blanket Sarella. After Xixivis was brought down and the Infernal army turned away, Morrow's Shield had brought a relative peace to the land, and the Arcanium went about its business at the bottom of the ocean.
In 55 ATC, a small scouting detachment from the Sandstorms, a mercenary group in the Dune Kingdom, found some odd crystals growing in a dead town. These crystals killed and reanimated many of the townsfolk, as well as turning regular crocodiles into hulking, lightning-throwing monstrosities. Many mercenaries were lost in the ensuing days, and the Dune Kingdom was unprepared for war. The Pharaoh could not gather enough soldiers to cover all of the fronts of war, and slowly but surely began losing towns, cities, and entire regions.
After losing most of the fertile southern shores of Feredas, Pharaoh Amon-Ra III marshalled his full might in the oases of Tet-Ran, hoping to set out and retake his lands. The first few weeks saw no resistance, which the soldiers took to mean that these were simple animals, scared away by the show of force. The Pharaoh remained wary, however, and was prepared for the counterattack: waves and waves of Risen, corpses raised by Arcanium corruption that retain full mobility and incredible strength. The troops stood fast, and their fortifications were enough to keep casualties to a minimum.
One night, during a particularly large assault on the Pharaohs lines, a new threat emerged and was seemingly coordinating with the undead: flying monstrosities never seen before, working alongside some particularly aggressive Harpies. The lines held, though casualties were high. All seemed quiet until the next night, when the army camped just outside the ruins of Kalahnu. That night was unnaturally dark, darker than it should be, even considering Olympia was entering a new moon and Infernia was a bare crescent sliver.
Gigantic though it was, no one saw the massive shadows descend upon the camp. Half a dozen towering humanoids made of shadow tore through the lines, tearing people apart and causing crippling fear in those trying to help. This time, however, the Pharaoh entered the fray and dismissed the shadows with a huge burst of sunlight, illuminating the monsters and inspiring his men to heroism. Whatever horrible intelligence guided these attacks was not happy.
The pharaoh disappeared a week later. Two weeks after that, the army was cleaning out a particularly rough section of Kalahnu, attempting to continue on even after their Pharaoh disappeared. Just after sundown, the Pharaoh appeared on the horizon, prompting many cheers and hopeful cries. Once he began to approach, however, the truth was shown: he was dead. His body was gray, partially mummified as though his burial had begun early and he was dragging a dead leg behind him, but still retaining many of his golden trappings. He summoned plagues of diseased animals just as a massive force of Risen attacked the lines.
The former Pharaoh wrought death and destruction on a scale no mortal had accomplished in known history, before just… disappearing. The survivors, roughly a thousand camp followers and wounded soldiers, fled north to Sapphire Pass to tell their tale.
After the Incident at Yalat Oasis and the subsequent fortification of Tartarus Hold, a few enterprising businesses began mining the crystal in earnest, sensing the potential it contained. This actually served to halt some of the spread of Arcanium, and allowed research to begin in depth. A plethora of uses were quickly found, discoveries that would change the face of daily life in Feredas. Firearms were developed and improved, farming implements were able to be empowered or automated, and in short order Arcanium took over all of the economies on Feredas, with unattuned Arcanium being worth twice its' weight in gold.
Precursor records show that Arcanium is native to Outerworld, though there is very little information as to why it came about in such a lightless environment. It behaves differently as well, acting as part of the environment as opposed to its' almost virus-like behavior on Sarella. Every being in Outerworld evolved alongside the crystal, and many seem to react poorly to being away from it.
Arcanium altered the very fabric of magic in Sarella, causing only those born with a symbiotic melding of Arcanium to be able to wield magic. Additionally, it seems to offer enhanced strength, endurance, and speed of healing. In everyone else, it seems to be almost parasitic, absorbing any beneficial magics cast at them.
The ever-present miasma that surrounds high-density Arcanium deposits seems to leak into the atmosphere, completely altering the weather patterns in the process. Somehow, Arcanium completely rewrote the natural weather systems of Sarella, resulting in the highly regular, but startlingly sudden seasonal changes after a Manastorm.
These storms seem to gain their odd powers through blending the edges of the Material, Ethereal, and Cognitive together, resulting in odd and often horribly violent events during a Storm.
Arcanium begins as an unattuned crystal. It will absorb the first magic cast at it, attuning itself to that magic type. Once attuned, a crystal cannot be un-attuned without extreme difficulty. There are ways to extract this magic via complex cages or housings, releasing either slowly over time to power items such as small lights or very quickly to create things like blast powder or magic wands. Additionally, it has been found that Arcanium can be used as a stand in for any consumable magic reagent, as long as it the crystal is unattuned.
Arcanium devices intended for long-term usage do need to be charged occasionally, but there are (expensive) ways to set up a device such that it draws power from the ambient arcanium in the area, allowing it to recharge on its own.
The most common usage is now in blast powder, used to bring firearms to the world of Sarella. A small bit of Evocation magic is cast into an unattuned crystal, then it is carefully ground up and combined with other materials to create a combustible powder. The most useful invention was the ArcLight, a streetlight that needs to be recharged once every 2 weeks. A small crystal is attuned to a powerful light spell, then a casing is set up to release the power little by little, resulting in relatively bright and steady streetlights.
The biggest Arcanium creation to date is the Imperial Railway, a train that runs on carefully coordinated levitation-attuned crystals.
Arcanium spread to Ragoth much slower than it did to Feredas, and has seemingly halted its' spread on that continent. There are rumors that the Dragon Lords have put some sort of countermeasures in place down in the Black Mountains, but no one has been able to find evidence of such a thing. Either way, the Arcanium is spreading much slower once it passes the mountains and the Black Courts mining expeditions are keeping it well under control, as well as reaping the benefits of its' trade. The low volume of Arcanium produced relative to the population of Ragoth has kept the Courts armies reliant on mostly using traditional non-imbued weapons and armor. Through a not-yet-understood process, Ragoth's weather has been unaffected by Arcanium,at least on a large scale. Odd localized weather phenomenon are occasionally reported by explorers in the Red Zone.
Some lucky few, on average around 10% of the population, are born with a sort of symbiotic relationship with Arcanium. They are unaffected by the ambient magical radiation and are far above their peers in almost every aspect.
The Blended are to everyone else what toy dog breeds are to dire wolves. Blended have the ability to use magic and receive magical healing, they can take far more physical and mental punishment than they should, and they have extraordinary regeneration. Many blended are able to summon reserves of strength with as little as an hour of rest, healing many minor wounds they received prior. Additionally, their reserves of physical and magical strength return at an accelerated pace; whereas many soldiers would be sore for a week following a battle, a Blended is ready to fight the next morning. Similarly for mages, where they would need to study for years or decades to achieve a measure of magical skill, a Blended grasps the nuances much quicker and can channel the Cognitive Realm much more efficiently. In short, Blended are stronger, faster, and smarter than their peers.
Non-blended have a very difficult time grasping any sort of magical ability, if they can channel any at all. As a side effect, non-blended also seem to be unaffected by healing or restorative magics, though destructive magics seem to have an undiminished effect. Many at the Imperial Academy theorize that the small bits of Arcanium in un-Blended bodies actually serve as a sort of parasite, absorbing any beneficial magic cast on them.
The Empty: Occasionally, a non-Blended will be born as an Empty, also called an Immune or a Void, and be completely disconnected from all magic. These beings are exceedingly rare but dangerous, as they are completely immune to all magic and seem to extend a small radius of null-magic, completely negating all magic near them. Arcanium-enhanced food crumbles to dust near them, magic is instantly nullified in their presence, and any Arcanium-infused materials cease functioning until they are removed from the aura.
Green Zones: Relatively unaffected, regular crop growth possible barring unlucky seasonal shifts. Atmospheric Arcanium dust is rarely present and Blended have a much lower birth rate here.
Yellow Zones: Tainted by Arcanium, but still habitable. Most crops and livestock are mutated slightly, sometimes to the benefit of those living there. These zones are usually self-sufficient, though experience a high rate of Blended births, plagues, famine, and various other early causes of death.
Red Zones: Uninhabitable, alien-looking wasteland. All that make their home here are monstrous Arcanium creations, and the occasional well-guarded mining venture. It is said some particularly mutated Blended or wandering Voids will make their homes in these secluded areas, but these rumors have never been substantiated.