alarming
impressive
Herculean
abstruse
appalling
arduous
astonishing
astounding
awe-inspiring
awesome
awful
brutal
burdensome
challenging
complex
conspicuous
critical
dangerous
daunting
delicate
demanding
difficile
difficult
difficulty
dire
direful
doughty
dread
dreaded
dreadful
earnest
effortful
egregious
exacting
exceptional
extraordinary
fabulous
fantastic
fatal
fateful
fearful
fell
freaky
frightening
frightful
ghastly
ghoulish
grave
considerable
hostile
powerful
terrible
biddable