(Diabolical) Late Latin diabolicus, derived from the Latin diabolus, "devil" -- of the Devil or devils; wicked, cruel, or fiendish. Diabolism is dealings with the Devil or devils by sorcery or witchcraft, or worship of such creatures; the character or condition of such a develish being. Diablerie is similar, but includes lore about the Devil.

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"

extremely evil or cruel; expressive of cruelty or befitting hell; "something demonic in him--something that could be cruel"; "fires lit up a diabolic scene"; "diabolical sorcerers under the influence of devils"; "a fiendish despot"; "hellish torture"; "infernal instruments of war"; "satanic cruelty"; "unholy grimaces"