A flame, which oft, they say, some evil Spirit attends, hovering and blazing with delusive light, misleads the amazed night-wanderer from his way to bogs and mires, and oft through pond or pool; there swallowed up and lost, from succour far.
~ From Paradise Lost 9.631-642
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 travelers 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 travelers 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 explanation isn't sufficient and there 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.
Some regional legends say these lights are wandering spirits of the dead, the work of devils (or youkai), or the pranks of fairies. They are feared by some humans as an omen of death. In other parts of the world, there are folk beliefs that supernatural fires appear where treasure is buried. With modern day scientific advancement, these "ghost lights" are also thought to be related to UFOs.
The term will-o'-the-wisp comes from wisp, a bundle of sticks or paper sometimes used as a torch, and the name Will, thus meaning 'will of the torch'. The term jack-o'-lantern (meaning Jack of the lantern) originally referred to a will-o'-the-wisp. In the United States, they are often called spook-lights, ghost-lights, or just simply orbs by folklorists and people who are into paranormal matters.
Will-o'-the-wisp in Paradise Lost has a metaphorical meaning, describing a hope or goal that leads one on but is impossible to reach, or something one finds sinister and confounding. In Book IX of Paradise Lost, lines 631-642, Satan is compared to a will-o-the-wisp when he leads Eve to the Tree of Knowledge of good and evil.
Will o' the wisp makes its appearance in the first chapter of Bram Stoker's Dracula: the Count, masquerading as a coach driver, takes Jonathan Harker to his castle in the night. When Harker asks Dracula about the lights, the Count makes reference to a common folk belief about the phenomenon by saying that they mark where treasure is buried.
In J. R. R. Tolkien's The Lord of the Rings, will o' the wisps are present in the Dead Marshes outside of Mordor. When Frodo Baggins and Samwise Gamgee make their way through the bogs the spindly creature Gollum tells them not to follow the will o' the wisps. He tells them that if they do, they will keep the dead company and have 'little candles of their own', indicating that these ghostly fires are not friendly.
In JK Rowling's Harry Potter series will o' wisps are called Hinkypunks.