I heard from someone else who played this that they are not playable, is it true? They seem to me the most interesting faction, if they are unplayable is there another faction that benefits from spreading the fog or can otherwise live in it that is playable?

I ask because I've been trying to find a strategy game with corruptible terrain that has gameplay impact since seeing civ4 fall from heaven 2 where one can play as the side spreading the corruption.