The Valley is believed to be the site where an ancient deity or fiend(which is a matter of some debate) is said to have fallen after being slain in some war on the outer planes. Very few settlements exist within the valley, mostly being populated by adventurers, fugitives, refugees, and other down and out sorts who prefer the isolation.
There are many things that stand out about the valley to any visitor, but the first thing most notice is the mushrooms. There are no actual trees in the valley. Instead, massive mushrooms grow up hundreds of feet tall. In fact, mushrooms are generally considered the dominant form of "vegetation" in the valley.
The second thing visitors notice is the fog. The valley is blessed with an overabundance of hot springs that leave the valley. These hot springs, combined with the enormous height of the valley's outer edge results in the valley having a temperate rainforest climate. With all the steam generated by the pools, combining with loose dust(and the multitude of wind-borne spores), a person on the floor of the valley never sees daylight or stars.
That doesn't mean however that the valley is actually dark. During the day, the sun does shine, but it's the gray lighting of a rainy day(and it's almost always raining). At night, the "forests" come alive with a multitude of colors thanks to the bioluminescence of the many "plants." This light is bright enough for a human to only see a few feet ahead of himself(think very dim lighting or moderate levels of fog), but not enough to hinder creatures with light sensitivity.