Source : Webster's Revised Unabridged Dictionary (1913)

Mephistophelian \Meph`is*to*phe"li*an\ (? or ?), a.
   Pertaining to, or resembling, the devil Mephistopheles, ``a
   crafty, scoffing, relentless fiend;'' devilish; crafty.

Source : WordNet®

mephistophelian
     adj : showing the cunning or ingenuity or wickedness typical of a
           devil; "devilish schemes"; "the cold calculation and
           diabolic art of some statesmen"; "the diabolical
           expression on his face"; "a mephistophelian glint in
           his eye" [syn: {devilish}, {diabolic}, {diabolical}, {mephistophelean}]