The Grey Pestilence

This isn't just a disease. It's the Morph-Plague—a mystical virus that doesn't kill, but rewrites living flesh according to the blueprints of other worlds.
Stage 1 (Blight): Skin turns grey and develops chitinous growths.
Stage 2 (Breaking): Bones lengthen, changing the shape of the body, turning peasants into animals or insect-like creatures.
Stage 3 (Mind): The person ceases to be themselves. They hear the "Song of the Gray Sky" and become obsessed with building crystalline hives.

The action takes place on a kingdom isolated by a continent, shrouded in fog. Ships neither come nor go.

1. Mountain Bastion (Ironhold): The last stronghold of humanity. The healthy live here, but food is running low. The Iron Duke rules, burning the infected in the square.
2. The Dead Forest: The trees here pulsate, and gray mold replaces leaves. This is the habitat of the "Whisperers"—the infected who retain their intelligence for the sake of cruelty.
3. The Sunken Cathedral: A city submerged by melting glaciers. Winged forms of the plague nest in the bell towers.

Feudalism has been destroyed. A person's value is determined by their "Resistance" (immunity). The lowest castes are the "Carriers" (people with minor mutations), who are used as slaves or living water filters. The highest are the "Inquisitors," who wear lead masks to avoid inhaling spores.