After the third invasion realms were pretty quick to re-claim the regions lost. The regions held by the invaders also had large numbers of Daimons in them so the monsters/undead that spawned there were destroyed by the Daimon Militia rather than wandering into human held lands.
Mesh would have continued to push on the northern Daimon lands but after our failed attack it seemed impossible to capture the area. The Daimons just brought more troops through the portal..
There was a storyline that the Daimon's regions around Beluaterra after the 3rd invasion were their breeding grounds but I don't recall that carrying over into the 4th invasion. I'm not sure if thats because of Mesh and other realms actually had destroyed them all or if it was dropped?
