Incidentally, the difference between Devil’s Nests and semi-Devil’s Nests is the extent of Mana pollution in the soil.

Lands with Mana pollution so severe that normal plants and animals living there would turn into monsters and monsters would spontaneously appear are referred to as Devil’s Nests. The presence of produce that only grows in Devil’s Nests, such as Kobol fruits, is also a requirement of being classified as a Devil’s Nest.

In order to convert a Devil’s Nest into normal land that doesn’t produce monsters, forests need to be cut down and swamps need to be filled in, things that can easily contain Mana must be removed and clergymen from the Church are needed to purify the land.

Semi-Devil’s Nests, on the other hand, are simply lands that multiple monsters have begun living in for whatever reason. For those making use of these lands, they are essentially the same as Devil’s Nests, but once the monsters are all hunted down, no more monsters appear. Afterwards, they can be used just like any other forest and marshland.
With this logic, Talosheim and its surroundings was a splendid Devil’s Nest, with its crops turning into monster plants. But nobody minded.