severity vs difficultness

severity and difficultness both are nouns.

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

As nouns, difficultness is a hypernym of severity; that is, difficultness is a word with a broader meaning than severity:

severity: something hard to endure
difficultness: the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result

Other hypernyms of severity include difficulty.

severity (noun)	difficultness (noun)
excessive sternness	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
used of the degree of something undesirable e.g. pain or weather
something hard to endure
extreme plainness