Death's Ride (Dungeons & Dragons module CM2, Adventure for Character Levels 15-20)

A King's Commission leads to danger! All communication with the barony of Twolakes Vale has ceased. King Ericall, worried about the security of his border and angered at the loss of tax revenues, has commissioned you, a delegation of powerful characters, to investigate.
This is not a petty problem to be solved by the armies of local nobility. Indeed, the king's forces are desperately needed elsewhere. Twolakes Vale is, after all, only a small barony on the far frontier of Ericall's domain. And yet...
The cloud is there, its nature and cause unknown. None have returned from the barony for weeks. Furthermore, the cloud is spreading, and vague reports of unrest and mysterious disappearances are starting to trickle in from nearby baronies.

Can you discover the secert of Twolakes Vale?