cornucopia of sorts - a device that emitted a steady stream of a yellow nutritious sludge. The Old Native town fed their large herds of domestic animals with this device, but when the Great Darkness swept over the land the town and the device were abandoned. Their irrigation system broke down, and the area reverted into a swampland. However, while the device was soon covered by sediment, it still continued to produce the sludge which spread out and gives the small lakes dotting the swam a yellow color.
The spread-out sludge allows the swamp to be far more verdant than it would normally be, and a large number of fungoid creatures have moved out of the caverns in the hills to the north and settled here. Some of them, like vegepygmies, fungal creatures, and myceloids are sapient and have developed their own societies in the swamp, and they may help or hinder explorers, though all of them will protect their homes fiercely. There will be Fungal Creature versions of human Inland Tribals and adlets, and possibly even a marsh giant or two. They will be active during the warmer months, but during the winter they will enter a state of hibernation - they (and other ambulatory plant creatures) will bunch up and let a special type of fungus mat grow over them and form a "shell" that protects them from the cold outside. In the spring, this shell will be discarded, and many such shells can be found at that time before their components are absorbed by other life forms. Unfortunately, this also makes them defenseless against external predators, especially the marsh giants of the neighboring swamps.
Nonambulatory giant mushrooms also grow here, especially near the center of the swamp where the device is located - which is also where the Old Native ruins can be found. PCs might find a number of Old Native artifacts there, though most of useful stuff has probably been discovered by the fungal creatures and put to use. The center might serve as a neutral "meeting ground" between the various intelligent plant life of the area, and perhaps even has some plant spellcasters - certainly a druid and possibly even an old, self-taught wizard - specializing in plant magics, of course.