horrendous vs frightening

horrendous is not a noun while frightening is a noun.

Word	Noun	Adjective	Verb	Adverb
horrendous	No	Yes	No	No
frightening	Yes	Yes	No	No

As adjectives, horrendous and frightening are synonyms defined as:

Other synonyms of horrendous include awful, dire, direful, dread, dreaded, dreadful, fearful, fearsome, horrific, terrible.

horrendous (noun)	frightening (noun)
	the act of inspiring with fear

horrendous (adjective)	frightening (adjective)
causing fear or dread or terror	causing fear or dread or terror