A fun one too! Well, interesting at the very least. Sort of collected a lot of my good ideas from way back and found an application for them.
It is basically this Earth-like world, where history went quite similar to that of ours. Until the time came for the War to End All Wars! And unlike the one in our world it really did stop all wars. Mostly because it never ended, like it did in our world. The bloodshed continued for years and years, cities destroyed, fields turned to wastelands, the countries that were fighting devastated and the reasons for war long forgotten. Eventually everything disappeared, as the world was covered by the sea of fog. The few remaining people have hidden in fortified cities, with practically no connection, as in the fog strange dark shadows roamed, oblivious to the humans. Those who went into the sea of fog, where the shadows were, never returned. Their body and mind were clouded and they became possesed. The only defence that people knew from the fog and the shadows was salt. Salt was used to make circles around people, when the fog and shadows surrounded them. Eventually rings of salt surrounded entire towns, letting the people behind their walls live in peace, in their small hoems among the ruins of the old world.
But above the sea of fog roam the witches. They fly their strange machines, so similar to those of the old world. The authority of the witches is absolute. They are summoned as the judges, the priestesses, the voice of reason. For they enforce peace among the surviving settlements. And those who dare oppose them are doomed to burn alive, as the defiant towns are destroyed by the witchflame, raining from the night sky. For it is in the sky where they live.
They rule the skies, but on the land, the true power belongs to the Guild of Salt Miners and Merchants, known commonly as the Guild. They are the ones who get the salt from deep beneath and salt is the lifeblood of this world. They have no fear of the witches, but mutual respect upholds peace.
Such is this world. Remnants of mankind hiding in the shadow of the past.