adj. 25B6; adjective
a fiendish torturer : WICKED, cruel, vicious, evil, malevolent, villainous; brutal, savage, barbaric, barbarous, inhuman, murderous, ruthless, merciless; dated dastardly.
a fiendish plot : CUNNING, clever, ingenious, crafty, canny, wily, devious, shrewd; informal foxy, sneaky.
a fiendish puzzle : DIFFICULT, complex, challenging, complicated, intricate, involved, knotty, thorny.