The Fallow Mire is a quite vast location, where you mainly encounter all sorts of the undead and hostile barbarians. It is fully optional to visit this location. This means that it is not required to visit this location to push the story forward. Personally, I recommend against skipping this location, because you can obtain lots of experience, Influence and Power Points here, and you can also obtain many valuable items here. Still, you need to realize that many of the monsters here may pose a big challenge for a beginning party.
The way to unlock this location: On the war map, select the "Rescue Soldiers Missing in Ferelden" operation and spend 8 Power Points to complete it.
Recommended experience: I recommend against going to the Mire before your party reaches, at least, level 9-10. Optimally, it is best to reach level 12, at least, thanks to which you will not have too much trouble killing the undead, nor with dealing with the barbarians in the local fort.