demon vs fiend

demon and fiend both are nouns.

Word	Noun	Adjective	Verb	Adverb
demon	Yes	No	No	No
fiend	Yes	No	No	No

As nouns, demon and fiend are synonyms defined as:

Other synonyms of demon include daemon, daimon, devil.

As nouns, demon and fiend are synonyms defined as:

Other synonyms of demon include devil, monster, ogre.

demon (noun)	fiend (noun)
an evil supernatural being	a cruel wicked and inhuman person
a cruel wicked and inhuman person	a person motivated by irrational enthusiasm (as for a cause)
someone extremely diligent or skillful	an evil supernatural being