Arch-fiend is a term used to describe a powerful demon or evil spirit. Antonyms for the word can include terms that describe the opposite of evil, such as angel, saint, hero, or savior. These terms imply a sense of goodness and light, which is diametrically opposed to the darkness and malevolence associated with an arch-fiend. Other antonyms could be used to describe creatures or beings that are less powerful or less sinister than an arch-fiend, such as imp, sprite, or fairy. These words suggest a smaller scale of mischief or magic, rather than the grand scale of evil that an arch-fiend implies.