Hades vs underworld

Hades and underworld both are nouns.

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

As nouns, Hades and underworld are synonyms defined as:

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

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