Mephistophelean /(? [or] ?)/

Meph·is·to·phe·le·an

Mephistophelean

prop. a.
  1. 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.