1) Devilish 2) Diabolic 3) Evil 4) Extremely evil or cruel 5) French word used in English 6) Mephistophelean 7) Mephistophelian 8) Wicked Found on https://www.crosswordclues.com/clue/diabolical
• (a.) Pertaining to the devil; resembling, or appropriate, or appropriate to, the devil; devilish; infernal; impious; atrocious; nefarious; outrageously wicked; as, a diabolic or diabolical temper or act. Found on http://thinkexist.com/dictionary/meaning/diabolical/