Every trapper and bog farmer living near marshes or swamps has his own name for these faintly glowing balls of light—jack o' the lanterns, corpse candles, walking fires, pine lights, spooklights, rushlights—but all recognize them as dangerous predators and false guides in the darkness.

Evil creatures that feed on the strong psychic emanations of terrified creatures, will-o'-wisps delight in tempting gullible travelers into dangerous situations. In the wild lands where they're most common, will-o'-wisps favor simple tactics like positioning themselves over cliffs or quicksand where they can easily be mistaken for lanterns (especially if they can set their traps near actual signal lanterns), allowing them to lure unwary travelers into perilous situations. On rare occasions, will-o'-wisps seeking easier pickings will move into a city and take up residence near gallows or follow along invisibly behind an army in order to harvest the fear of the dying men; why the vast majority choose to remain in the swamps where victims are scarce remains a mystery. Will-o'-wisps only use their electric shock ability under extreme duress, preferring to let other creatures or hazards claim their victims while they float nearby and feast.

Will-o'-wisps can glow any color they choose, but are most frequently yellow, white, green, or blue. They can even vary their luminosity to create patterns—many will-o'-wisps are fond of creating vague skull-like shapes in their glow to further terrify their victims. Their actual bodies are barely visible globes of translucent spongy material 1 foot across and weighing 3 pounds, capable of emitting light from every surface. A will-o'-wisp's light is approximately as bright as a torch, and though they do not seem to use sound to communicate with each other, they hear perfectly and can vibrate their bodies rapidly to simulate speech.

While vilified by most other sentient creatures, will-o'-wisps are actually quite intelligent, if utterly alien in their reasoning. Sometimes organized into groups called “strings,” their society and goals remain complete unknowns to outsiders, as do their origins, though they have sometimes been known to strike bargains with those who can offer them vast quantities of appropriately frightened victims. As will-o'-wisps do not age, and are effectively immortal unless killed by violence, particularly ancient will-o'-wisps can serve as fantastic repositories of knowledge from the ancient past, although convincing one of these cruel creatures to cooperate with an interrogation can be a trick in and of itself.