Will O' The Wisp are a type of fairy or ghost described in the folklore of Great Britain and Ireland, though regional varieties exist in other cultures - they were believed to be evil spirits or "impure" souls that haunted marshes at night, leading unwary travellers into dangerous areas where they would become lost or even drown, appearing as balls of light the Will O' The Wisp often showed signs of intelligence and was a silent trickster feared by travellers who had to travel alone at night.
In recent times Will O' The Wisp has been explained by science as a product of swamp gas and active imaginations but as with all things supernatural there are still people that believe this explaination isn't sufficient and their are some places that still fear this malevolent spirit.
As well as being a famous troublemaker in folklore Will O' The Wisp is a popular character in fiction but is not always depicted as being evil or dangerous.