adjective

extremely cruel or unpleasant.

fiendish methods of torture

wicked,cruel,vicious,evil,nefarious,unspeakable,brutal,brutish,savage,barbaric,barbarous,beastly,inhuman,murderous,bloodthirsty,ferocious,ruthless,heartless,pitiless,merciless,black-hearted,unfeeling,malevolent,malicious,villainous,malignant,devilish,diabolical,hellish,demonic,satanic,Mephistophelian,ungodly,odious,base,dastardly