fiendish

fiendish
    adj 1: extremely evil or cruel; expressive of cruelty or
           befitting hell; "something demonic in him--something that
           could be cruel"; "fires lit up a diabolic scene";
           "diabolical sorcerers under the influence of devils"; "a
           fiendish despot"; "hellish torture"; "infernal
           instruments of war"; "satanic cruelty"; "unholy grimaces"
           [syn: {demonic}, {diabolic}, {diabolical}, {fiendish},
           {hellish}, {infernal}, {satanic}, {unholy}]
    

Fiendish \Fiend"ish\, a.
   Like a fiend; diabolically wicked or cruel; infernal;
   malignant; devilish; hellish. -- {Fiend"ish*ly}, adv. --
   {Fiend"ish*ness}, n.
   [1913 Webster]
    

77 Moby Thesaurus words for "fiendish":
      Draconian, Mephistophelian, Tartarean, animal, anthropophagous,
      atrocious, bad, baleful, barbaric, barbarous, beastly, bestial,
      bloodthirsty, bloody, bloody-minded, brutal, brutalized, brute,
      brutish, cannibalistic, cruel, cruel-hearted, cursed, damnable,
      demoniac, demoniacal, demonic, demonish, demonlike, devil-like,
      devilish, diabolic, diabolical, evil, execrable, fell, feral,
      ferocious, fiendlike, fierce, ghoulish, heinous, hellborn, hellish,
      infernal, inhuman, inhumane, malefic, maleficent, malevolent,
      malicious, malign, malignant, monstrous, murderous, ogreish,
      outrageous, ruthless, sadistic, sanguinary, sanguineous, satanic,
      savage, serpentine, sharkish, sinister, slavering, subhuman,
      truculent, unchristian, uncivilized, ungodly, unhallowed, unhuman,
      vicious, wicked, wolfish