Meph·is·to·phe·li·an 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.