hellish vs fiendish

hellish and fiendish both are adjectives.

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

As adjectives, hellish and fiendish are synonyms defined as:

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

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

hellish (adjective)	fiendish (adjective)
extremely evil or cruel; expressive of cruelty or befitting hell	extremely evil or cruel; expressive of cruelty or befitting hell
(informal) very unpleasant