Daemons are, to me, a really evocative villain class. They're the evil that even other inhabitants of the Lower Planes find revolting. They're also, per my subjective assessment, not over-used as villains in published materials. I was pondering a setting where the daemons are basically in charge of a prime material world, and what that would look like.

Some questions for brainstorming

-How did the daemons take over? Was it a war? Did they subvert another religion and launch a crusade?
-What unique races, if any, exist in this world? I had some inspiration from here .
-What kinds of non-daemonic kingdoms still survive in a marginal way -- are they well-concealed tribes, armored arcologies, etc?
-What are the daemonic plans for this world now that they have largely taken it over (perhaps this also raises the question of their original aims, which, admittedly, might have changed)?
-What do the daemonic polities look like -- are there four main realms, each devoted to one of the Horsemen, or some other arrangement?
-What kinds of unique/distinctive mechanical subsystems might operate in such a world.
-What kinds of environmental changes might have followed the daemonic incursion?

That's all I have for now -- looking forward to seeing any responses!