will-o'-the-wisp vs phantasy

will-o'-the-wisp and phantasy both are nouns.

Word	Noun	Adjective	Verb	Adverb
will-o'-the-wisp	Yes	No	No	No
phantasy	Yes	No	No	No

As nouns, phantasy is a hypernym of will-o'-the-wisp; that is, phantasy is a word with a broader meaning than will-o'-the-wisp:

Other hypernyms of will-o'-the-wisp include fancy, fantasy, illusion.

will-o'-the-wisp (noun)	phantasy (noun)
an illusion that misleads	imagination unrestricted by reality
a pale light sometimes seen at night over marshy ground	something many people believe that is false
	fiction with a large amount of imagination in it