diabolic vs fiendish

diabolic and fiendish both are adjectives.

Word	Noun	Adjective	Verb	Adverb
diabolic	No	Yes	No	No
fiendish	No	Yes	No	No

As adjectives, diabolic and fiendish are synonyms defined as:

diabolic and fiendish: extremely evil or cruel; expressive of cruelty or befitting hell

Other synonyms of diabolic include demonic, diabolical, hellish, infernal, satanic, unholy.

diabolic (adjective)	fiendish (adjective)
extremely evil or cruel; expressive of cruelty or befitting hell	extremely evil or cruel; expressive of cruelty or befitting hell
showing the cunning or ingenuity or wickedness typical of a devil