infernal region vs underworld

infernal region and underworld both are nouns.

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

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

Other synonyms of infernal region include Hades, Hell, netherworld, Scheol.

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