seriousness vs severeness

seriousness and severeness both are nouns.

Word	Noun	Adjective	Verb	Adverb
seriousness	Yes	No	No	No
severeness	Yes	No	No	No

As nouns, severeness is a hypernym of seriousness; that is, severeness is a word with a broader meaning than seriousness:

seriousness: the quality of arousing fear or distress
severeness: used of the degree of something undesirable e.g. pain or weather

Other hypernyms of seriousness include badness, severity.

seriousness (noun)	severeness (noun)
the trait of being serious	extreme plainness
the quality of arousing fear or distress	excessive sternness
an earnest and sincere feeling	something hard to endure
	used of the degree of something undesirable e.g. pain or weather