Terrifying is an adjective. Adjectives describe something, for example the blue house or the terrifying lion. So, small would also be an adjective.
Suddenly is an adverb, which is a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree, etc. For example, the wind blew gently.
Hope this helps!