GreatLemur said:
I like my mythological appropriations with the serial numbers filed off, and my magic to work like science.
In this sense I would describe fey and fey-like creatures as the original "attractive nuisance." They are a nearly irresistible temptation that is almost invariably dangerous, in keeping with TwinBahamut's note that 99% of the time a human/fey encounter results in misfortune for the human.
This would cover a good few of the more popular conceptions of fey. You could group the trooping fairies with their courts, dances, and feasts, the "belles dames," the selkie-wives, the mermaids, the nymphs, the rusalkas, and the kitsunes under it easily. From my, admittedly limited, understanding, these stories tend to run along the lines of, "Man sees beautiful fey woman, falls hopelessly in love, and either dies or lives to regret it."
I think the idea of attractive nusiance could also be stretched to cover some of the uglier fey like the leprechauns, and various fairy-tale witches. These fey have something that is irresistible like a pot of gold or a gingerbread house. It would also cover some fey animals, like the water horse, a wild horse so beautiful many try to mount it and ride it home, only to be drowned when the beast dives into the nearest lake.
The idea kind of runs aground on things like redcaps, I admit.
This perilous beauty could be distilled into a "Nature's Lure" that would be analogous to the Obyrith "Form of Madness" ability. Using the Belle Dame as an example, the knight would have to make a Will save vs. the Belle Dame's Nature's Lure power or "sojourn here, alone and palely loitering" until he dies of exposure. Like Form of Madness, Nature's Lure would be tailored to the fey creature in question. It would be important, IMHO, that Nature's Lure be an "always on" ability to reflect the danger that the fey pose, even when their intentions are of the best.