Bitter Waters is a d20 System scenario for 1st- to 3rd-level characters. Part of AEG's Adventure Keep line of short, cheap modules in half-sized format (4-1/4"x11"), it is intended to be run as a single session. Intended for use with the Dragons sourcebook. From the back cover:

Through acrid mists and bitter waters they march. They are quiet, making no sound as they leave the marshlands. Spears held high, the warriors scan the fog for signs of the attacker.

Every step took them further from their old lands.

Every step brings them closer to the lands of their "allies", who had abandoned them when the marsh turned black and foul.

Every step brings one of them down to the befouled earth beneath the water, there to rest forever.

Every step churns up more of the poisoned water as it seeps between their scales and below their skin.

So many had already died on this march, and after the attack, they had so few left...

The mists part in the night. The village's light bathes them in a false welcome. The human guards are unsteady, either from poison or from drink. The town celebrates the end of the trade season, but not a man raises a toast to the creatures that died for it.

So many had died from the first attack of the beast and from the aftermath, but the humans would suffer for such treachery....