EzekielRaiden
Follower of the Way
Really, you could even do a both/and situation.Defend swamp from ancient evil or encroachment via civilization seems obvious.
There is an ancient evil here. AND there's an up-and-coming, albeit far more banal, evil exploitative outside civilization as well.
The tempting thing is to turn to one side or the other to try to protect yourself, only for that to really be a choice of which evil overlord you deal with: an ancient corrupting force or the callous cruelty of far-away colonizing powers. But heroes with gumption and guile may find, or create, a third option that doesn't place dark powers or imperial jerks in control--perhaps even playing one evil against another, then exploiting the winner's weakened position to kick them out for good.
I think this has some pretty good motivating value, and it kinda turns the usual adventure concept on its head. You ARE the "barbarians", but you're also the ones fighting back against the darkness. You can have simple-evil villains (the ancient evil and its cultists), and also complex-evil villains (the actual people of that far-away empire, who can be much more sympathetic than their imperial overlords). There can still be dungeons and the typical wilderness trekking, but the scary things are imperial camps (effectively war camps!) where few to no civilians would be expected to appear.