fearsome(frightening, especially in appearance) adjective [UK: ˈfɪə.səm] [US: ˈfɪr.səm]
formidable(causing fear, dread, awe or admiration as a result of size, strength, or some other impressive quality) adjective [UK: ˈfɔː.mɪ.dəb.l̩] [US: ˌfɔːr.ˈmɪ.dəb.l̩]