diabolical

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”