grimness vs rigor

grimness and rigor both are nouns.

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

As nouns, grimness and rigor are synonyms defined as:

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

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