Crimson is covered with an insidious thick red fog that gives the planet its name. It rises two miles above the planet's surface, sends most life into a helpless stupor, and corrodes machines from the inside. Just above the fog hovers massive machines kept aloft by powerful psionic flows emanating from the planet beneath. Strange gears four hundred meters across slowly turn in the red skies, while the factories attached to them use some unfathomable process to produce the most powerful psionic drugs in the grand cluster. Bolted onto these factories centuries later are a collection of homes and structures housing the modern population, a collection of native workers who can see the psionic flows emerging from below, occupying soldiers guarding the wealth of the planet, and doomed prisoners recovering from their exposure to the mist in the mines below.
On the surface itself, protected and isolated by the fog, is an alien ecology featuring forests of an organism shaped like coral and a host of small hardy creatures that thrive in the otherwise hostile fog. dotting this alien wilderness are the old ruins of vast cities made of smooth towers with mushroom tops, massive vents in the surface spewing out the fog, miserable mines where the enslaved criminals of the empire are forced to harvest exotic psionic drug predecessors, and the camps of the flappers, a hairless winged race with a great crests, long beaks, and a taste for human flesh.
Crimson was one of the most memorable worlds of my campaign Called From Exile, and I'm planning a campaign set entirely on it. It is visually memorable, hides immense secrets, and easily lends itself to intrigue, being isolated, hostile, and at the center of grand politics, all at the same time.