River Styx vs Styx

River Styx and Styx both are nouns.

Word	Noun	Adjective	Verb	Adverb
River Styx	Yes	No	No	No
Styx	Yes	No	No	No

As nouns, River Styx and Styx are synonyms defined as:

River Styx and Styx: (Greek mythology) a river in Hades across which Charon carried dead souls

River Styx (noun)	Styx (noun)
(Greek mythology) a river in Hades across which Charon carried dead souls	(Greek mythology) a river in Hades across which Charon carried dead souls