infernal vs fiendish

infernal is a noun but fiendish is not a noun.

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

As adjectives, infernal and fiendish are synonyms defined as:

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

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

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

infernal (adjective)	fiendish (adjective)
being of the underworld	extremely evil or cruel; expressive of cruelty or befitting hell
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