A rivulet of the great Atharâg river once flowed through this area. Fungal growth from the direction of Crulock began clogging the western flow around the start of the Lith-Crillion Era. This magically enhanced vegetation grew fast and thick, expanding eastward along the river banks and into it. By the end of the era it had forced the flow of the rivulet to cut a new course; with it doubling back to its source forty miles to the north and east. The fungal growths were stopped by this change in flow, where it now forms a half-circle of tremendous water flow. The strength of the current in such that it tears the vegetation apart as it creeps into the river. Any that does take hold is scraped, burned, or hit with plant-killing magic by a group of druids serving the Nature Protectorates. This group views the magically enhanced vegetation as a threat to the area and is charged with keeping it contained in the Mephirouth Trench.
There are all sorts of Underdark plant-life in the dried riverbed, along its banks and into the vast cavern near its center. Most of these are harmless. The dangerous types are gas spores, phantom fungus, shambling mounds, tendriculos, yellow musk creepers, snapper saws, strangle vines, and a host of other predatory plant-life. The verdant fungals of Mephirouth Trench are also home to things that feed on the plants, like giant beetles, flies, and centipedes and all the things that prey on them all the way up the Underdark food chain.
The myconids also have a considerable presence in this area. They are scattered and in small groups numbering in the thousands. An expedition into the interior by Aredhel, before she became a god, resulted in findings that say these groups remain small and scattered because of something called the Mephirouth Rot. This plant disease is named after the primordial Mephirouth who once resided at Crulock. Mephirouth Rot causes the brain of sentient plants to slowly rot resulting in insanity and for some a permanent confusion like the wizard spell of the same name. After a couple large colonies were wiped out from this disease, the myconids of the area decided to keep their populations small and far apart from each other. From time to time, they join together and eradicate effective colonies, often calling in outside mercenaries like fire mages and pyromancers.