fiendish vs infernal

fiendish is not a noun while infernal is a noun.

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

As adjectives, fiendish and infernal are synonyms defined as:

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

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

fiendish (noun)	infernal (noun)
	an inhabitant of Hell

fiendish (adjective)	infernal (adjective)
extremely evil or cruel; expressive of cruelty or befitting hell	being of the underworld
	characteristic of or resembling Hell
	of or pertaining to or characteristic of a very uncontrolled and intense fire
	expletives used informally as intensifiers
	extremely evil or cruel; expressive of cruelty or befitting hell