Monolith of the Underworld

From the introduction:

While wandering through the open grassland, the party will come across a sharp boundary where at one point the land is fertile and the next is a dried barren waste. This boundary is a fine line and can be seen curving in the distance (actually a circle radiating outward from the monolith). If the party looks, a small insect can be seen crossing into the wasted section. Within minutes the insect goes into convulsions.

User summary:

For mid-level characters.