severity vs grimness

severity and grimness both are nouns.

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

As nouns, severity and grimness are synonyms defined as:

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

severity (noun)	grimness (noun)
excessive sternness	something hard to endure
used of the degree of something undesirable e.g. pain or weather	the quality of being ghastly
something hard to endure
extreme plainness