fiendish vs demonic

fiendish and demonic both are adjectives.

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

As adjectives, fiendish and demonic are synonyms defined as:

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

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

fiendish (adjective)	demonic (adjective)
extremely evil or cruel; expressive of cruelty or befitting hell	extremely evil or cruel; expressive of cruelty or befitting hell