hellishness vs frightfulness

hellishness and frightfulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
hellishness	Yes	No	No	No
frightfulness	Yes	No	No	No

As nouns, frightfulness is a hypernym of hellishness; that is, frightfulness is a word with a broader meaning than hellishness:

hellishness: The property of being extremely bad, miserable, difficult or terrifying; resembling a nightmare.
frightfulness: the quality of being frightful

hellishness (noun)	frightfulness (noun)
The property of being extremely bad, miserable, difficult or terrifying; resembling a nightmare.	the quality of being frightful