underworld vs mythical place

underworld and mythical place both are nouns.

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

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

underworld: (religion) the world of the dead
mythical 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, imaginary place.

underworld (noun)	mythical 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