Mephistophelian

prop. a. Pertaining to, or resembling, the devil Mephistopheles, “a crafty, scoffing, relentless fiend;” devilish in character or aspect; crafty; showing cunning or ingenuity or wickedness; as, a mephistophelean glint in his eye.
Syn. -- devilish, diabolic, diabolical, mephistophelean. [ 1913 Webster ]