The Will o' the Wisp is an interesting thing to study, possibly a ghost or a fairy from one view and a swamp phenomena by another, one that may be explained by scientific means but whose folklore persists. There is debate even today about whether the Will o' the Wisp is supernatural or a natural occurrence and explanations for it include both optical illusions as well as spontaneous ignition of swamp gasses. There is also a rich array of folklore around it which offers many explanations of it from that viewpoints as well as stories of dealing with it
In recent times it has become less common for people to see Will o' the Wisps, and many accept the scientific explanation although science itself has never been able to reproduce or measure them successfully. When the phenomena appears it can be as small as a candle flame or as large as a torch, pale or bright, and the light will reflect off of nearby objects (Sanford, 1919). Colors can vary and may include green or white and the phenomena has been seen passing through windows and doors and inside buildings although it is most commonly seen over or near water, particularly swamps. Explanations for what causes it include bioluminscent plants and animals, gases given off in the process of decay, and bubbles of plasma, although no single theory can or has been proven (Drudge, 2016).
In folklore the Will o' the Wisp has many different names which are indicative of the folklore attached to it. The common name of Ignis Fatuus is Latin for 'fool fire'. It is also known variously as Bill-with-the-wisp, Hobbledy's Lantern, Jack-a-lantern, Jenny-with-the-lantern, Jenny-burnt-tail, Peg-a-lantern, Joan-in-the-wad, Kit-in-the-stick or Kitty-candlestick, Kitty-with-the-wisp, the Lanternman, Pinket, Friar Rush, Gyl Burnt-Taylf, Hinky punk, and Hobby Lantern (Briggs, 1976). It's possible that like so many other types of fairies we are not looking at one specific being but rather a range of beings who all fall under the umbrella term of 'Will o' the Wisp' because of how they appear and what they do. In that case any being who shows up in the dark of night bearing a light to mislead travellers could be called a Will o' the Wisp even if we also know it as another distinct being such as the Pwca.
The nature of the Will'o'the'Wisp can be either mischievous or malicious depending and they have been known to both lead travellers harmlessly astray and also to lead them to their deaths. They do this by appearing as lights in front of lost travellers; as the traveller follows the light the light moves and leads them astray. In the case of the mischievous spirits this may mean into a ditch or in circles but for the dangerous ones it could mean off a cliff or into a bog where they drown. They are also known to attack people directly in some folklore, physically chasing them, driving them mad with a touch, or causing a burning sensation on the bottoms of the feet (Ashliman, 2016).
The Will o' the Wisp is often explained as a human spirit of some sort that has been cursed to wander by night bearing a light. the purpose of this light also varies and depends often on why the spirit is cursed to wander. In some areas of Scotland it was said to be the spirit of a girl who had died and spent her afterlife searching the area near the shoreline for a plant used in dyeing cloth; and that she did so because she'd been too greedy in hoarding the dye when she was alive (Ashliman, 2016). In other stories, for example, it was someone who illegally moved boundary markers or cheated neighbors and is set to wander with a light to show where the true boundary is. In the Netherlands and parts of Germany there is a belief that Will o' the Wisps are spirits of unbaptized children who will approach people and try to lead them to water hoping to be baptized (Ashliman, 2016). They can be dealt with by either offering them baptism or throwing graveyard dirt at them.
By other accounts though the Will o' the Wisp is a fairy. In Wales both the Ellydon and Pwca take on the role of the Will o' the Wisp, leading travellers astray. Stokes describes one such incident with the Pwca here: "[A] peasant who is returning home from his work, or from a fair, when he sees a light traveling before him. Looking closer he perceives that it is carried by a dusky little figure, holding a lantern or candle at arm's length over its head. He follows it for several miles, and suddenly finds himself on the brink of a frightful precipice. From far down below, there rises to his ears the sound of a foaming torrent. At the same moment the little goblin with the lantern springs across the chasm, alighting on the opposite side; raises the light again high over its head, utters a loud and malicious laugh, blows out its candle, and disappears up the opposite hill, leaving the awestruck peasant to get home as best he can."(Stokes, 1880). In parts of Germany they are viewed as a type of gnome who can help lost travellers if petitioned to do so and paid for their help, but who will also lead astray those who annoy them (usually by seeking them out); protections against them include walking with one foot in a wheelrut (Ashliman, 2016). In another German story they are described as having wings and flying, and one appeared to attack a girl while she walked because she was singing a song which mocked the spirit (Ashliman, 2016).
There is a distinct crossover as well between the two beliefs, that the Will o' the Wisp is human spirit and that it is a fairy, which we see in many versions of the Jack o Lantern story. In that classic tale, generally viewed by folklorists to fall into the auspices of Will o' the Wisp lore, a person makes a deal with the devil but outwits him by some means and eventually finds himself turned away from both heaven and hell alike. Left to wander in the cold darkness between worlds after a time he finds a light or is given one, which he uses to light his way. In a version of the story related by Stokes in 1880 the reader is explicitly told that the man, having been turned away from both afterlives, was turned into a fairy (Stokes, 1880). This is reinforced by Danish lore which states that a Jack o Lantern is the soul of 'an unrighteous man' and that one should never call on him or point him out if you see him but that turning your cap inside out will protect against him, which is true to fairylore (Ashliman, 2016). This may reflect wider beliefs that fairies themselves are those who belong to neither heaven or hell, something we see in both narratives about the fairies origins as fallen angels and also some beliefs that relate dead humans as fairies.
The Will o' the Wisp is an intriguing and unusual fairy - or spirit - one of the few that science has sought to explain and also one of the more well documented as a phenomena. I have never seen one myself, but my husband has once in the swamp behind our home. Are they natural phenomena? Ghosts? Fairies? I think perhaps the answer is all of the above.
Sanford, F., (1919) Ignis Fatuus; Scientific Monthly vol 9 no 4 https://www.jstor.org/stable/6287?seq=2#page_scan_tab_contents
Briggs, K., (1976) A Dictionary of Fairies
Ashliman, D., (2016) Will-o'-the-Wisps
Sikes, W., (1880) British Goblins
Drudge, C., (2016) A New Explanation for One of the Strangest Occurrences in Nature: Ball Lightning