What lies under the turf deep in the earth of that grassy mound? The PCs would very much like to know...
Inspired some years ago for a Kult/CoC Halloween game
Knowledge is Power. That is, he who makes the knowledge, wields the power.
In one region of a forest, all of the trees are identical, down to the leaves and the twigs. If the PCs carve something into one of the trees, it could mystically propigate until it covered all of the trees, or could vanish since it was not carved into the one true tree.