To set the stage, the stars were aligned in all the wrong signs. Everyone new something bad was going to happen. When winter set in hard and early, they thought that was it.
They were wrong.
The Ducks and Geese were flying south early on. Their population had swelled over the last five ideal years. The sky was thick with them, as were the ponds. People were taking them down to stock their larders and to have one more round of fresh meat before the winter.
Centuries later, the pieces were put together. At the time, nobody knew. The Ducks and Geese were the carriers. It was carried in their feces and their bodies. Those that hunted the creatures, were near the ponds or resting places, handled the bodies improperly, or -for a few- ate the flesh were the first infected. They passed it on to others.
It appears to be your basic minor flu. The victim is achy, warm, their eyes water, their nose runs, they cough, and they might have minor tremors. The precursor symptoms last for about nine days, plus or minus for the constitution of the victim. It then appears the target gets better. They feel fine, though unbeknownst to them, they were still infected. And they could infect others.
Note: Spell Caster could not cast spells because of their symptoms and seem to hit them harder than most
Everything seemed normal. The early flu with the hard onset of winter, with everyone being inside or together, seemed perfectly natural - to be expected. Though the Flu did seem to hit The Demi-Humans hard, it was not too unusual. Some of the harder hit, the mystically inclined, were physically frail, so it seemed expected. It all seemed in line with the ill-alligned stars.
An infected person was nearly symptom free for three to four weeks. Many thought they were completely cured.
The symptoms start as before, but with an increase in shaking. The Palsy is an infection in the para-sympathetic nerve channels of those who can channel magic. While a few frailer mundanes have symptoms, only those with latent or manifest mystic channels are effected. The channels cause feedback, preventing any spell casting and causing the physical nerves to misfire. The energy backups that result exacerbate the flu like symptoms. The longer the symptoms persist, the worse they all get. Death occurs from respiratory distress, manna block fever, or sheer exhaustion, within three weeks, but in as few as three days.
The results were devastating. In the Human Population, 98% of the wizards and mages died that winter. The few that didn’t were mostly ascetics and hermits (almost all of them died within the decade). The latents are hard to calculate, but given the lack of Gift in the population after the winter, it was 75% fatal. Even those of clerical persuasion were targetted, expiring at a 55% rate. This created the power vacumn that changed the balance of powers and made way for the various reformations and revolts that occured in the next three decades.
In the Elventi Population, with its inherent magical nature, the disease was only 60% fatal, 100% fatal among the adepts. However, it effected the birth rate. Those affected, but survived, became infertile at a 98% rate. (Magic is part of the Elventi reproductive ritual.) This is the reason for the Final Elventi Exodius so they could die at sea into the setting sun.
The Dwarventi Population should of been immune given popular wisdom. However give the close quarters that Dwarventi live plus the magical resistance and metal empathy abilities (which are magical), the death rate was 99%. The population was too diffused to successful continue resulting in the extinction of the Dwarves.
The Goblin and Orcenti races, while not magical, seemed succeptable to the physical symptoms. Given the squallor they normally lived in, they kept infecting, mutating, and re-infecting themselves. Entire tribes and nests were wiped out due to sickeness and fever induced homicide.
The world became a very different place that winter. The world became Mundane.
This would make an interesting campaign background. You could start play the previous year or so, the watch the results of the Omen Winter. Society will change radically, and in such chaos PCs could be beacons of hope or grease that makes the wheels roll.