fiendish vs diabolic

fiendish and diabolic both are adjectives.

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

As adjectives, fiendish and diabolic are synonyms defined as:

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

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

fiendish (adjective)	diabolic (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