(a.) Difficult to overcome or accomplish; as a formidable task\; exciting fear or apprehension; impressing dread; adapted to excite fear and deter from approach, encounter, or undertaking; alarming.

Herculean abstruse alarming appalling arduous astonishing astounding awe-inspiring awesome awful brutal burdensome challenging complex conspicuous critical daunting delicate demanding difficile difficult dire direful dread dreaded dreadful earnest effortful egregious exacting exceptional extraordinary fabulous fantastic fatal fateful fell freaky frightening frightful ghastly ghoulish grave grim grisly gruesome hairy hard hard-earned hard-fought heavy hideous horrendous horrible horrid horrific horrifying imposing impressive incredible indomitable intimidating intricate jawbreaking knotted knotty labored laborious macabre marked marvelous mean menacing mighty mind-blowing mind-boggling morbid no picnic not easy notable noteworthy noticeable of mark onerous operose outstanding overwhelming petrifying portentous powerful prodigious redoubtable remarkable rigorous rough rude rugged schrecklich serious set with thorns severe shocking signal sober solemn spiny staggering steep strenuous striking superior terrible terrific terrifying thorny ticklish toilsome tough tremendous tricky unbelievable uncommon uphill weighty wicked wonderful