underworld vs imaginary place

underworld and imaginary place both are nouns.

Word	Noun	Adjective	Verb	Adverb
underworld	Yes	No	No	No
imaginary place	Yes	No	No	No

As nouns, imaginary place is a hypernym of underworld; that is, imaginary place is a word with a broader meaning than underworld:

underworld: (religion) the world of the dead
imaginary place: a place that exists only in imagination; a place said to exist in fictional or religious writings

Other hypernyms of underworld include fictitious place, mythical place.

underworld (noun)	imaginary place (noun)
(religion) the world of the dead	a place that exists only in imagination; a place said to exist in fictional or religious writings
the criminal class