Mephistophelian
Concise Oxford English Dictionary © 2008 Oxford University Press:
Mephistophelian /ˌmɛfɪstəˈfiːlɪən, mɪˌfɪstə-, ˌmɛfɪstɒfɪˈliːən/ (also Mephistophelean)
▶adjective wicked; fiendish.
– origin C19: from Mephistopheles, an evil spirit to whom Faust, in the German legend, sold his soul.
'Mephistophelian' also found in these Oxford entries:

