I think it is important that the affected regions are wastelands, not blighted. We do not want walls and bottlenecks. They should probably adversery effect joined regions as well, in order to discourage their use on own border regions.
They could start with ill omens that the priests discover. An adventurer could then investigate and find the cursed item. The item grows in power and yields significant bonuses. At later stage it begins to signal imminent doom and then wastes the region.
The holy items do the same, but at the end heal the region. They are seeded only when noble density requires more land.
