I don't think the spawn rates should relate to what a region is (townland, mountain, city, ...), but to the geographical location. Think the Huns invading Europe. Think triggering a gigantic movement of people as the world itself has seen in the late roman/early medieval period (4th-6th century). If you wish, be entirely arbitrary in where you make this happen. I would start by not caring about people complaining because you will always find people complaining. Just decide: "This eastern bit of this continent will have to migrate or die." Then get started.
If you want you can spread this out over pretty much all continents, and choose different points of "invasion" for each continent. Some continents which you would prefer to see vanish as opposed to others you could give a higher spawn rate, i.e. making it harder for the entire continent to survive. Because, you know, when all lands are rogue the only thing left to do is migrate - which will in return populate a different continent and allow it to survive these invasions.
The only big downside I see on this is that the "migrating" people would be forced to be absorbed into the power structures of the continent they migrate to. That may be unfair. But as suggested earlier you could arrange it so that entities of continent A get a place to live on continent B. Heck, after a while the refugees from continent A could just fulfil their destiny and carry on the invasion on continent B - without the need of monsters or undead.