fiendish vs unholy

fiendish and unholy both are adjectives.

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

As adjectives, fiendish and unholy are synonyms defined as:

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

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

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