will o' the wisp

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.

(British folklore) A flickering light or lantern, especially over bogs, swamps or marshes, held by a supernatural creature who uses it to lure travelers at night to their demise.

(figurative) A delusionary or otherwise unachievable goal that one feels compelled to pursue.