The phenomenon sometimes referred to as "Will 'o Wisp" is a light that teases the traveler by staying just out of reach. The Will 'o Wisp has several causes. Most well known is its use by the more mischievous Fae in misleading travelers. Leading the unwary astray is one of their more widely shared past-times, used for play or malice by the lesser and to discourage trespassers by the greater Fae.
One of the reasons why naked force is ineffective against the Good Neighbors is that they take steps to prevent any aggressors from ever finding them in the first place.
More disturbingly, there are accounts of elves themselves lead astray by the lights. While this could be ascribed to a lesser Fae being tricked, there is a story? of the Woad Walker? itself becoming lost like this. Is there something else that causes the lights that the Fae themselves merely learned to imitate?
The original document is available at http://www.thereamid.com/tiki/ElfLight