SECRET VILLAGES OF FUGITIVE SLAVES DEEP WITHIN THE SWAMP. Do I have my next Civil War Cthulhu scenario? Yes, yes I do.
Heck yeah. This is a fantastic setting starter.
Also the swamp has a huge circular lake in the middle of it that is not fed by any streams. Scientists -- and Indian legends -- theorize that it might have been created by a huge underground peat fire. Or maybe a meteorite strike.
Seriously, you can't make this stuff up.
This detail in and of itself is Mythos material. I recall reading a modern story about a Mythos tale around a lake that was not fed by any streams. It was fed by some sort of underground fissure/aquifer with something unnatural below that. Can't recall the title or author unfortunately. Might have been Barron's “The Redfield Girls”, but I don't think so.
Neil Gaiman's "American Gods" also features a lake with a sinister purpose in the latter quarter of the book.
Make the black runaway slaves the players. Make the problem be in the swamp and the ex-slave village can't go anywhere for help, because "ex-slave". Really turns the desperation and isolation up to 11. Add some slave hunters as road-blocks/trouble to keep the real problem from being solved and you have a scenario.
I like the idea of having the player team be slave runaways coming to the village in expectation of freedom.
First, this concept gives the players a significant reason to be/work together. Secondly the GM (I know it's Keeper but I'm going with GM for simplicity) can divide useful skills among the group of escapees/refugees because it's likely that only a group of people with some degree of synergy and competence managed to survive for this long. In addition to skill interdependence the players have on one another, the GM could also introduce player flaws (physical disadvantages like Ethan's last game) that the entire party has to come together to overcome (because Bob the guy who lost part of a leg in a cannon blast is our only doctor damn it!) which ratchets up the tension.
Make the problem be in the swamp and the ex-slave village can't go anywhere for help, because "ex-slave". Really turns the desperation and isolation up to 11.
I agree with this. Playing a group of fugitives turns the desperation and isolation.
Add some slave hunters as road-blocks/trouble to keep the real problem from being solved and you have a scenario.
You could do this but it seems a bit too ... expected to me. I do like the idea of slave hunters a road-blocks (increasing isolation) preventing the players from escaping the Mythos location, but I don't like the idea of slave hunters as front and center antagonists. It's been done before.
How about this. The Rock and a Hard Place situation. Players are refugees seeking respite and freedom from slavery. In the back of the mind is the nebulous fear of being re enslaved.
However the slave village location they are fleeing to is just as unsavory because it has a monster/cultist problem. The players expect rest and a breather, this expectation is ripped from them, horror begins as they realize they are out of the frying pan and into the fire. Perhaps a fatigue mechanic would be useful here. First act is a fleeing/escape scene. Second act arrival at village/ investigation/ rising horror. Third act resolution or desperate flight out of swamp dodging cultists and then trying to dodge slavers waiting for them.
Or the party comes to the slave village and find it abandoned with signs of violence. And they have to stay the night there with a monster somewhere on the loose because slaving parties are looking for them outside the swamp. Then it turns into the classical "it's a rainy night and we can't go outside" haunted house.
Alternatively again, perhaps the party are new ex-slaves to the village. Since they are new (expendable) the "village elder" tasks them with investigating why slaves are disappearing from the village (it's a supernatural threat).
Lot of options here. Looking forwards to what you come up with!
Edit:
The Great Dismal Swamp, now reduced by draining and development, is managed as a federal wildlife refuge. The once-notorious panthers are gone, but bears, birds, deer and amphibians are still abundant. So are venomous snakes and biting insects. In the awful heat and humidity of summer, Sayers assures me, the swamp teems with water moccasins and rattlesnakes. The mosquitoes get so thick that they can blur the outlines of a person standing 12 feet away.
Oh damn. You need to come up with a mechanic for obscuring/blinding swarms in the swamp.
Also all those snakes. Voodoo/Pentecostal snake handlers gone bad? The party's only ally is a crazy one of these in a old isolated shack?