1) Alarming 2) Appalling 3) Dire 4) Direful 5) Dread 6) Dreaded 7) Dreadful 8) Fearful 9) Formidable 10) Frightening 11) Horrendous 12) Scary 13) Terrible 1) Alarming 2) Awful 3) Dire 4) Direful 5) Dread 6) Dreaded 7) Dreadful 8) Fearful 9) Frightening 10) Horrendous 11) Horrific 12) Intimidating 13) Like ogres and A-bombs 14) Ogrelike 15) Really scary 16) Redoubtable 17) Scary 18) Terrible 
• (a.) Frightful; causing fear. • (a.) Easily frightened; timid; timorous.
Fear'some adjective 1. Frightful; causing fear. [ Scotch] 'This fearsome wind.' Sir W. Scott 2. Easily frightened; timid; timorous. 'A silly fearsome thing.' B. Taylor