Today, it's all about will o wisps, those little balls of light that like to make adventurers go for a swim in the local swamp. A very long swim. According to the 2e D&D Monstrous Manual, they feed off of the electrical activity of panicked folks. So, of course, these malevolent entities want the death to be slow and horrifying if at all possible.
So where does this creature originate?
The D&D example given above is clearly based off of the spontaneous flammability of P2H4 + PH3 + O2, resulting in an ignition that consumes CH4 as fuel.
Let's try that again.
Organisms associated with the decomposition of biomass undergo a process caused methanogenesis, which produces methane. In the right circumstances, decomposition probably also gives off phosphines. In the right combinations, exposure to oxygen causes the phosphines to ignite, and the methane burns for a little bit, resulting in eerie lights in a swamp (which has abnormally high decompositions of biomass).
One more try.
In a swamp, dead things give off stuff that goes boom. This freaks people out.
Central European mythology most closely matches the description most of us are familiar with - i.e., supernatural spirits that try to attract the attention of travelers, making them want to leave the road and go exploring to their deaths.
Since this is a worldwide phenomenon, though, it's worth looking at what other cultures have done to explain these little fireballs of doom.
In Venezuela, for instance, they are considered ghosts of a particularly psychotic conquistador named Lope de Aguirre, self-attributed "Prince of Freedom", who was chopped into little pieces and scattered around the region after a brief reign of terror.
In Wales, there are stories of lights that turn out to be lanterns held by dark and sinister creatures that like to lead peasants to dangerous precipices before turning off their light (oddly similar to Poes in Zelda, I might add).
In Japan, they were thought to be human souls, at least in some stories. Here's a nice picture.
|Well, apparently this is a Japanese interpretation|
of a Russian will'o'wisp, but it's still cool.
There's a couple of stories that inspire me the most.
Redefinition Idea #1
A Will'o'wisp is a spirit - a form of incorporeal undead in some games - that hovers over a long-buried tomb. It cries out to be remembered - for being forgotten is the greatest eternal torture of all. Rumors have it that these tombs, if they can be found with digging, have forgotten secrets and ancient treasures.
It might not be as simple as digging deep enough with a shovel, though. Sometimes these tombs are purposely made to be forgotten, and magic - perhaps necromantic or divinatory in nature - could be required to let the earth itself remember the tomb exists, making it once again visible to enterprising adventurers.
This combines the ghosts/souls aspect from some cultures, with legends in Scandinavia and Briton that said will'o'wisps indicated buried treasure - not unlike Leprechauns. One of those legends required digging with a dead man's hand.
Redefinition Idea #2
The Will'o'wisp is actually a bioluminescent low-altitude flying lizard, capable of lighting up so brightly that during the night and at a distance, it simply appears as a bright globe. This is a defense mechanism - the creatures use this capability to protect their territories, either by tricking trespassers away or, even better, luring them to dangerous spots where they can become prey. These creatures are classified as "aggressive scavengers".
This idea is based off of fringe groups that seem to think pterodactyls still exist in the modern world, with a splash of aboriginal lore on will'o'wisps (or "min min") - courtesy of Australia.