The First Age and the Early Shogunate
The history of the World of Steel And Glass begins in the days of the Shogunate, at the end of the First Age. At this time, the Shogunate was a realm united in theory, but divided in practice; endless disputes between the various satraps of the Shogunate, combined with an ineffective Shogun who was reluctant to use the power that he had access to, resulted in a land torn with strife and turmoil. The Sidereals remained hard at work repairing the damages caused to the land by the Usurpation - their tireless efforts were stabilizing the world, but they had no time to unite the Dragon-Blooded. The Lunars, by contrast, were reorganization and exploring their changed natures, building footholds in the edges of Creation, but as of yet content to remain there, waiting to see how the wars of their inferiors would resolve.
The Contagion served to change everything.
As the officers and leaders of the Shongunate continued their feuds, unsuspecting of the danger brewing, the Deathlords gathered together to enact a great ritual, creating a deadly disease that began to spread from shadowlands across the world into Creation. Slowly, the Great Contagion began to pick up steam, as it infected humans, animals, and plants, corrupting everything in the world.
It should have been the end of all things, a plague that would have killed nearly everyone in Creation. In the places that it sprang forth, nine out of ten beings died, and the shadowlands bloomed. But the power of the plague was stymied, thanks to the efforts of an old Twilight-Caste Solar by the name of Kallir Alaric.
Alaric lived on the borders of the world, where he had been hiding for over five hundred years after his Exaltation. It was a fluke chance that he had evaded the frequent Wyld Hunts that had been sent to destroy him when he had first Exalted, and he had even then only escaped by faking his own death at the hands of a Fair Folk noble and escaping into the Wyld, where he remained for a hundred years, honing his skills. Considering himself ready to see how the world had changed, Alaric had then left, and for two hundred years more had travelled the fringes of the world, always on the move, never resting in case his secret was discovered. And it was thus that he encountered the Contagion early in its spread, as he entered a village filled with the dying.
Alaric, as it happened, was a skilled medic, and when he discovered the Contagion, he developed a Charm to fight it - sacrificing his life to spread a counter-Contagion through the world that would grant everyone immunity to it, a cure which he gifted, as he died, to a few trusted mortal followers. Each of his followers began to travel, covering the four corners of the world, and as each walked, every being that they met was granted the cure in turn, and every being that came into contact with those was cured, until the cure had spread to every living being in Creation, overwhelming the Contagion and starving it until it ceased to be.
No one knew why the disease stopped almost as soon as it started, least of all the extremely frustrated Deathlords, but it was clearly stopped, and it appeared that Creation had narrowly averted calamity. In the fringes of the world, entire towns were wiped out, and a few regions were devastated, but the overall death toll was barely 15%. Instead of ending the world, the Deathlords had merely unleashed a plague deadly enough to draw the attention of the entire Shogunate.