friar's lantern, ignis fatuus, Also called: jack-o'-lantern a pale flame or phosphorescence sometimes seen over marshy ground at night. It is believed to be due to the spontaneous combustion of methane or other hydrocarbons originating from decomposing organic matter
2.
a person or thing that is elusive or allures and misleads
[C17: originally Will with the wisp, from Will short for William and wisp in former sense of a twist of hay or straw burning as a torch]