underworld vs fictitious place

underworld and fictitious place both are nouns.

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

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

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

Other hypernyms of underworld include imaginary place, mythical place.

underworld (noun)	fictitious 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