I had a bit of an alien incursion scenario - mentioned here, Let's flesh it out a bit...
Standing Stones of Leth:
The stones are scattered throughout the Lowlands. Their original masons and erectors are a mystery, and the stones themselves are not native to the country, appearing to have been transported a great distance. Scarred and exfoliated by lightning strikes, these stones glow pale blue under a full moon.
In spite of their allochthonous and mysterious origin, several settlements have grown up around the stones in the belief that they provide some sort of protection or good fortune. Indeed, gardens and orchards appear to grow more verdantly in the vicinity of these stones.
The stones were placed for more alien reasons, to allow entry into this world by ephemeral shades. Lightning strikes upon the stones provide enough impetus to temporarily open a gateway to a dimension of incorporeal beings, the Shades of Leth. During a storm, 1d6 shades will pass through these temporary gates into our world.
A possessed human will have the blue glow of the stones in their eyes. A captured vessel is 'ridden' by the shade in its inscrutable mission. Anyone possessed in such a way will likely be exhausted and spent by a shade, requiring replacement every three to five days, wracked by dehydration and shock. An abandoned body, if still alive, will be mindless and hopeless for revivification.
Shade of Leth: 5HD, AC0 (AC2 if possessing human), Atk: 1d6+1, touch, (save or 1 CON loss, permanent), not undead, incorporeal resistances (resistances transfer to the possessed body), destroying the shade also kills the host body. Save vs death (or equivalent) is required to avoid possession (CON bonuses apply). A shade loses 1HD per hour outside of a host, dissipating after four hours.
The shades, once crossed over into this world, attempt to convene upon a central location in the Lowlands - a massive, buried hearth stone of the same composition as the standing stones. When sufficient shades have arrived, the stone will arise, allowing not only shades, but their home dimension to spill through, wracking the world in unreality and making it a hospitable colony for the shades.
So far, insufficient simultaneous lightning strikes have occurred to allow a critical mass of shades through to activate the core stone. However, there is rumor of misled or manipulated apocalypse cults intending to artificially create simultaneous lighting storms in the stones' vicinity...