A delusionary or otherwise unachievable goal that one feels compelled to pursue.
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.
will-o'-the-wisp
How often have the words will o' the wisp and will-o'-the-wisp occurred in a corpus of books? (source: Google Ngram Viewer )