Demons in the dark
Demons in the dark is all about being lost and trying to find your way back home. Some themes are taken from movies like Pitch Black and Aliens, while other parts are just so characters can get a crack at some iconic D&D baddies.
One of the important things to remember in this campaign is the lack of safety. There’s no safe resting spot, there no place to call home, there is no handy magic shop to resupply. Characters need a way to make/upgrade/acquire things, which means that your hardiest characters will suffer if they don’t put a little effort into something other than hitting people.
That’s because the second most important thing is that the Labyrinth is big. Really, really big. Bigger than can be easily measured using football fields. The Labyrinth is not the Underdark, but similar in scope—you could visualize it as a buffer of sorts…but mostly to keep people out of the Underdark, not the other way around.
There are several civilizations in the Labyrinth, most notably (at this point, anyway) the neogi. Slavers bent on proving their supremacy, respecting only strength, they can be fiercely competitive with the drow and illithid populations.
Most beings use some light down here, even if they do possess darkvision, because you never, never want to accidentally wake up a bullette or purple worm. The light disturbs them and their stirring will generally give advance notice to the party to Not Go That Way—whether you live there or not.
There’s your setting. Tactics are GM’s!