rigor vs difficultness

rigor and difficultness both are nouns.

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

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

rigor: 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 rigor include difficulty.

rigor (noun)	difficultness (noun)
something hard to endure	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
the quality of being valid and rigorous
excessive sternness