Blood Omen

From the introduction:

This is a stop-gap adventure which begins while the characters are traveling between destinations. It is evening and the sky has become overcast and stormy. The adventurers are pelted by rain and the clouds, moving almost supernaturally fast above them, are indicative of ill portent. The way ahead becomes increasingly misty, and visibility becomes greatly limited. Elves using infravision will detect nothing but grazing animals in the surrounding plains.