Marches of Madness
This is a young swamp, that formed naturally about 60 years ago. Before that, there where a lot of settlements in the area as it had a river delta with water that back than was drinkable.
Every day of travel roll once on this table:The toxins of the swamp have polluded the water and at places there have formed toxic gas pockets. These pockets differ wildly in what their effects are but most of them are unpleasant. The gass cloud has a 1d100+15 feet radius.