A Will-o-Wisp (known by many, many variations of that name, such as Will o' the Wisp) is a strange monster that takes the form of a glowing orb of light that tends to appear in dark forests or near swampy regions. Whilst sometimes portrayed as merely mischievous, more often the will-o-wisp is a malign trickster who uses curiosity (and perhaps a dab of mind-control magic) to lure victims into following it, invariably delivering them to danger in the form of a natural hazard or a monster.
Fun fact: There really is a Real World phenomena that matches the common folkloric "Will-o-wisp"; known as "ignis fatuus" (translation: "Fool's Fire" and not in fact a faux-latin insult for an obese pyromanic). The common modern explanation is that certain kinds of swamps bubble up various flammable gasses that ignite upon coming into contact with the air. In other words, the original "Swamp Gas" explanation.