From Dragon Eye Atlas
It is a natural depression, an entirely enclosed valley, that due to the local climate, prevailing winds and geography is permanently enshrouded in mist. From above, especially the steeper northern ledge, the view is phenomenal. Inside The Mist, however, visibility is low and any number of predators are looking for prey. Few who venture deep into the valley have returned.
The permanent mist cover starts at around 500 m altitude, and spans an area almost 20 km east-west and 13 km north-south. At this altitude, the mist is still more of a fog and visibility drops slowly, averaging at about 100 m. The forest here is wild and mostly untouched by humans, but otherwise not much different from the surrounding areas.
When you go below about 400 m altitude, the mist quickly becomes much more dense. This area covers about 12 km east-west and 8 km north-south. Visibility quickly drops to 50 m and then less. Predators become more common here, as they can use the mist to sneak up on their prey. Few who have gone this deep into The Mist have returned. Those who did report dense forest and strange animals, many of whom they never saw close (those who did probably did not survive).
There are rumours of ruins deep in The Mist, but none of those who returned have confirmed them, though some parties have been suspiciously silent on the question, evasive almost. Maybe they found something and don't want to share it?