grimness vs severity

grimness and severity both are nouns.

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

As nouns, grimness and severity are synonyms defined as:

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

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