horror vs fearfulness

horror and fearfulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
horror	Yes	No	No	No
fearfulness	Yes	No	No	No

As nouns, fearfulness is a hypernym of horror; that is, fearfulness is a word with a broader meaning than horror:

horror: intense and profound fear
fearfulness: an emotion experienced in anticipation of some specific pain or danger (usually accompanied by a desire to flee or fight)

Other hypernyms of horror include fear, fright.

horror (noun)	fearfulness (noun)
something that inspires horror; something horrible	the trait of being afraid
intense aversion	an emotion experienced in anticipation of some specific pain or danger (usually accompanied by a desire to flee or fight)
intense and profound fear