formidable -- (extremely impressive in strength or excellence; "a formidable opponent"; "the challenge was formidable"; "had a formidable array of compositions to his credit"; "the formidable army of brains at the Prime Minister's disposal")
formidable, redoubtable, unnerving -- (inspiring fear; "the formidable prospect of major surgery"; "a tougher and more redoubtable adversary than the heel-clicking, jackbooted fanatic"- G.H.Johnston; "something unnerving and prisonlike about high grey wall")
(Sound)
