rigor vs grimness

rigor and grimness both are nouns.

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

As nouns, rigor and grimness are synonyms defined as:

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

rigor (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