Is terrifying an adjective?
Yes, it is an adjective. It is the present participle of the verb (to terrify) and means very frightening.
The verb to terrify has participle adjective forms terrifying and terrified. The adverb form of the first is terrifyingly (in a manner that causes terror). * Originally, the adjective terrific applied to the noun terror. As such it had the adverb form terrifically, which now is used to mean extremely or intensely.