Elves

Elves, or Huldrefolk--literally "hidden folk" in Icelandic--are Germanic faeries often depicted as beautiful, majestic beings who wield devastating supernatural powers.  They're said to live within certain mounds, hills, and stones across England, Iceland, and Northern Europe, and leave their homes mainly to hold enchanted celebrations or go hunting on moonlit nights.  The homes of elves are regarded as sacred, and it is very dangerous to disturb them without the faerie residents' permission.  Even today, construction projects in places like Iceland either avoid elf-homes entirely or wait until suitable arrangements can be made prior to proceeding--efforts to appease the fey inhabitants can range from relocating their home to simply giving them time to move out, but either way any construction done near their homes without permission is likely to be afflicted by mysterious accidents and general ill-luck.  

Angry elves might shoot their victim with an invisible arrow, which can cause blindness or worse.  This is known as being elf-shot, and the best cure for it is to not offend any elves in the first place--which can be difficult if they happen to be lurking nearby unseen.  In general, it's best to never speak ill of elves, and one should refer to them only when absolutely necessary; and even then, use only pleasant pseudonyms such as "Fair Folk" or "Good Neighbors." 

Similarly to pixies, sudden rings of mushrooms are sometimes said to be the result of the elves' late-night revels; but another type of faerie ring said to be the product of elven parties is known as an elf circle, which is essentially a round patch of flattened grass--presumably formed by the elves' dancing.  Elven music is able to entrance a human to dance indefinitely, but if a person should happen upon the elves during one of their nighttime dances, they must not join the celebration--a few hours spent dancing in an elf circle can result in years passing outside of it.

So if you're travelling at night in an area known to be inhabited by elves, make sure to head in the opposite direction of any hauntingly beautiful music you might hear, and if you absolutely must comment on it, please make sure to be exceedingly polite when you do so.