I have the same question! I've facemurdered everything in the swamp (including Morion), and eliminated all of the deep pits. I've been wandering around it for several turns now, wondering what remains to be done there. There are still some troll huts, but I can't raze them because it's not my terrain, so I've just been repeatedly killing the trolls that spawn there. I even checked the other sides of the mountain ranges, but I've covered the entirety of the domain and there's just nothing left. But the quest is still insisting that I heal the scars or whatever.
The AI has taken the liberty of founding a million outposts just outside the borders now that the threat is gone, but I remain unable to found cities in the area.
I'm a Death Archmage who's corrupted about 1/3 of the shards on the map, so I'm able to kill everything mercilessly with just my sovereign. (Back up, start casting Dirge of Ceresei or Drain Life, do hundreds of damage to everything on the map).
