There exists a prestigious academy with a history going back hundreds of years. Graduates are highly skilled and very much sought after for employment. For purely traditional reasons, the academy requires its students to become proficient in a skill or area of knowledge that has no use in the modern world. This could be Ancient Egyptian, an antiquated form of writing such as shorthand, knowledge of building or repairing antique steam engines, programming COBOL, etc. PCs who graduated from this academy must be sure to have this skill in their repertoire.
An insidious creature, most likely somehow "related" to trappers and lurkers, the Dead Leaves (for no other name exists as of yet for this foul thing), hibernates for three of the four year's seasons, deep underground. Its active time is Autumn, when trees shed their leaves, depositing colorful carpets across the ground. The terror then emerges and blends in with the surrounding leaves, perfectly camouflaged, waiting patiently for unsuspecting victims. In appearance it resembles nothing more than a ten foot square, six inch thick, layer of bright yellow, orange, and red leaves. The only hint that someone is walking on top of it, comes in the form of an unusual amplified sound of leaves crunching underfoot. Too late usually, the victims notice this additional "crunch". The Dead Leaves will then swirl and "rise" up to smother and suffocate the victim, like a colorful, malevolent, boa constrictor.
Fire, as can be imagined, is particularly effective against this creature, but one has to *know* it's there before putting it to the torch. And there's the rub. The creature is impossible to "identify" in a large patch of fallen leaves by eyesight alone.