'Terrifyingly' is an adverb built straight from 'terrifying', and unlike 'terrifically' it has kept all its fear. It has two jobs. First, it describes how something is done or how it looks, meaning 'in a way that makes people very afraid': the storm came terrifyingly close, the bridge swayed terrifyingly. Second — and this is how you meet it most often — it sits in front of an adjective and means 'so much that it is frightening': terrifyingly fast, terrifyingly good, terrifyingly accurate, terrifyingly young. Notice that even a good quality can take it; 'she is terrifyingly good at chess' is a compliment with a shiver in it. Writers also use it to open a sentence — 'Terrifyingly, no one noticed for three days' — where it comments on the whole fact, not on any one word. It is a strong word, so it stands out; use it when you really mean the feeling, and it is common in news writing, reviews, and storytelling.