Those areas are covered in permanent mist.
* The Stillmist up north holds an elven refuge which is inaccessable if you don't know how to find your way or get a guide (it's actually not there at all but in an adjacent world). If you go in there blindly you will probaby stumble around finding nothing until you stumble out again.
* The Shadowgate pass is an unstable area filled with ominous mist harbouring aggressive predatory demons – think something like the dangerous fog in the movie ”The Mist”. In practice inaccessible.
* The other two mist areas I can't really vouch for since I didn't put them there, but I suggest the northern one close to Belderand hold volcanic vapour rising from the ground, stinking of sulphur that is unwholesome to inhale. The one in the Thynde Range might be remaining Blood Mist or something altogether different.
Thanks for the quick reply, this is really good and adds some nice intrigue/world flavour, will it be included in the gm or ravens Purge books?