Mephistophelian
/ˌmefɪˌstɑːfəˈliːən/
Definition 1 of 1
Pro Tip 1/3Literary Origin
Understand that 'mephistophelian' has literary origins from a tale about a deal with a demon. 
The novel's antagonist displayed a mephistophelian cunning, reminiscent of his Faustian namesake.
Pro Tip 2/3Negative Connotation
'Mephistophelian' often implies a deeply negative judgment of someone's character. 
Her mephistophelian tactics during the debate revealed a lack of moral integrity.
Pro Tip 3/3Caution in Use
Be careful using the word, as it's strong and could be offensive if misapplied. 
Accusing him of a mephistophelian agenda was a step too far in the argument.