The story
From Will (“male given name”) + of + the + wisp (“a bundle of straw or hay used as a torch”).
Etymology from Wiktionary, CC BY-SA 4.0.
will o' the wisp
Today it means: Any of several kinds of pale, flickering light, appearing over marshland in many parts of the world with diverse folkloric explanations and multiple possible scientific explanations including bioluminescence and chemiluminescence. Full definition →