The swamp is an extremely dangerous place. You have to traverse by boat, deal hordes of undead and other monsters, plus disease is rampant. No sane person goes there. Luckily adventurers aren't particularly sane and there are some good monetary hooks to lure pc's there.
Several of the hexes have unique encounters. The hexes are 18 miles Those that don't can be populated by some fantastic random tables. The folk that live in the swamp are strange. Honestly, that's a gross understatement. I won't go into too much about the encounters but let's say my favorites are a dungeon that's located within the ossified remains of a huge beast. My second favorite encounter is related to this. It's referred to as The Corpse Pile and is a hex of undeath. Essentially it's a moving hex full of sodden undead things. Eighteen miles full of undead. Imagine if the BBEG from Death Frost Doom was able to add these to his horde... The hex affects those around it too.
The book ends with new diseases, several new monsters (full of gross swampy flavor), and two classes.
- Transfiguration Host - These folks are extremely odd. They are tied to the weird religion and traditions of the area. Essentially they have powerful parasites in them that seek experiences and mutate them. Stat was they have hit dice and saves like specialists, but use the fighter xp table.
- Shaman - These swamp folk have dangerous and useful spirits bound to them. They can send this bound spirit out. They can also use their power over spirits to make intangible spirits tangible. They have cleric hit dice, saves, and use their xp table.
This book is really well put together. It's a memorable and nasty setting. Yes it's technically written with Lamentations of the Flame Princess it could easily be adapted to any version of D&D. I love this book and think you will too. You can grab your copy here.