frightfulness vs horridness

frightfulness and horridness both are nouns.

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

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

Other hypernyms of frightfulness include awfulness, dreadfulness, terribleness.

frightfulness (noun)	horridness (noun)
the quality of being frightful	a quality of extreme unpleasantness