underworld vs infernal region

underworld and infernal region both are nouns.

Word	Noun	Adjective	Verb	Adverb
underworld	Yes	No	No	No
infernal region	Yes	No	No	No

As nouns, underworld and infernal region are synonyms defined as:

Other synonyms of underworld include Hades, Hell, netherworld, Scheol.

underworld (noun)	infernal region (noun)
(religion) the world of the dead	(Christianity) the abode of Satan and the forces of evil; where sinners suffer eternal punishment
the criminal class	(religion) the world of the dead