Noun
See Ignis fatuus.
Source: Webster's dictionarywill o' the wisp
Financial "synergy" is a will-o'-the-wisp. It looks good on paper, but it fails to work out in practice. Peter Drucker
I am nothing - a will-o'-the-wisp at your service - a clod of vivified dust whom its progenitors christened Amos Midwinter. I have no possession but my name, and no calling but that of philosopher. Naked I came from the earth, and naked I will return to it. John Buchan