underworld vs Hades

underworld and Hades both are nouns.

Word	Noun	Adjective	Verb	Adverb
underworld	Yes	No	No	No
Hades	Yes	No	No	No

As nouns, underworld and Hades are synonyms defined as:

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

underworld (noun)	Hades (noun)
(religion) the world of the dead	(religion) the world of the dead
the criminal class	(Greek mythology) the god of the underworld in ancient mythology; brother of Zeus and husband of Persephone