A word about dark forests.
Level 1-10 bogs randomly become level 1-10 dark forests.
Dark forests can be scouted and they can be attacked.
If a dark forest wins a battle, it still remains where it is.
The troops that were killed will not return. Only those surviving the battle remain in the forest.
Each time a dark forest is defeated, it disappears.
Once in a while (not sure if it's a set time or after a certain amount of dark forests have been defeated), new dark forests reappear in new places all over the domain (or maybe only one province at a time) to replace those that have been defeated.
Be careful if you hit dark forests during this period of regeneration, as they may regenerate as a higher level than before.
An error message saying "Cannot perform this action on target location" is a sign of this regeneration being in progress.
If you do not get a report from attacking a dark forest, the forest most probably isn't there anymore, but has reverted back to a bog.
Since defeated forests disappear, you can defeat any one dark forest only once.