The Demon's Mist

The Demon <name> made a final attempt to gain a foothold on Demons Pass, to do so he cast a magical spell to funnel gasses from the demon plane through the portal in Demons Pass. The downside to this spell is that it went out of control and the two worlds began to merge at the portal, new subspecies of demons were born on the demonic plane and demonlike creatures started appearing near the portal of the material plane.
The gas is most potent at the source, destroying all life, while at its further reaches life begins to decay. The initial geyser of gas caused a major earthquake and explosion in the mountan which exposed the inner cave system turning into a series of ravines and mesas. The only way for the common races to traverse the gas is to use a form of magical barrier; most commonly achieved by a magic spell or weilding an enchanted item.
The eruption of the mountain destroyed some nearby villages and the survivors banded together along with famed adventuruer Gideon to build a keep from the fallen debris.
As the gas began to expand beyond Demons Pass survivors worried that it may engulf the whole world. Whispers of an ancient artifact that could stop the gas started to circulate amongst the survivors.
Beyond the radius of debris from the fallen mountain lay a mystical forest. The trees growing there somehow had an innate magic that would protect them from the effect of the decaying gas. Some of these trees were harvested by Gideon and the surviors to give the keep resistance to the gas as well. While building the keep they used the forest as a refuge.
After prolonged exposure to the gas, some of the survivors started to develop mutations gaining some of the traits that the demons possess. This has allowed them to traverse the decaying gas over small distances for short periods of time.