rigour vs grimness

rigour and grimness both are nouns.

Word	Noun	Adjective	Verb	Adverb
rigour	Yes	No	No	No
grimness	Yes	No	No	No

As nouns, rigour and grimness are synonyms defined as:

Other synonyms of rigour include asperity, hardship, rigor, rigorousness, rigourousness, severeness, severity.

rigour (noun)	grimness (noun)
something hard to endure	something hard to endure
the quality of being valid and rigorous	the quality of being ghastly
excessive sternness