Wetlands

From publisher blurb:

This ninth in the series deals with Wetlands. It discusses the effects of thick mud, swamp gas, diseases and parasites, swarms of insects, rot, and all the things associated with one of the most uncomfortable environs imaginable.

Now, instead of simply traveling to a tomb of horrors, the DM can have it in the middle of an expansive bog, with clouds of biting flies and noxious gases. Instead of stopping at the local apothacary, the characters are forced to search a hundred miles of stinking marsh for a rare alchemical ingredient that's needed to save the life of the king's son.

Also included diseases, parasites, a new character background, new equipment, a new language, a terrifying new creature, and random encounters.