unholy vs fiendish

unholy and fiendish both are adjectives.

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

As adjectives, unholy and fiendish are synonyms defined as:

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

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

unholy (adjective)	fiendish (adjective)
not hallowed or consecrated	extremely evil or cruel; expressive of cruelty or befitting hell
extremely evil or cruel; expressive of cruelty or befitting hell
having committed unrighteous acts