From the Introduction
Disturbing tales of dead warriors walking the Undermarch reach the eyes of the adventurers. The wards that keep undead out are intact, and yet the dead walk. The adventurers are hired to clear out the undead, and yet might find themselves surprised by what they find.
OK, here's the story with fungaloids. One day you are passing a shaded grove in the Queen's Wood or a dank cave in the Owl Barrens or an abandoned mining tunnel near Forge and you notice some mushrooms. No big deal, right? A month later on your way back you are attacked by some small animated fungus creatures. You kill them and burn the fungus. A year passes and you are headed that way again, only this time there is a whole town of fungal creatures, there are fungal farmers and fungus shepherds and fungal soldiers and even fungal beasts pulling carts. You give the area a wide berth and when you finally get back home you head straight to the people in charge to warn them... only to find well-dressed fungal diplomats about to sign a treaty that hands over a good chunk of your territory to them.
Don't worry, it is a 'peace treaty', they assure you that they aren't invading... with these new official borders peace between 'fleshmen' and fungaloids is assured. By the next week fungus is growing all over town...