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.
Near a major city, a mirror image of the place has appeared. This strange double is infested with hostile insectoid humanoids that appear to match the city folk in numbers and armaments. The humans’ diviners have determined that the secret of the city’s appearance can be found within the double’s college of magic, so heroes are needed to explore within this mirror city and discover what has drawn it from its distant realm. There isn’t much time to spare, as the bug folk find their mammalian counterparts unnatural and blasphemous, and plan to destroy them.