How to Say "fiends" in Spanish
The most common Spanish word for “fiends” is “diablos” — use 'diablos' when referring to people who are exceptionally wicked or cruel, often with an implication of being almost demonic or supernatural in their evil. It's commonly used in contexts involving a stark contrast between good and evil..
diablos
/DYA-blohs//ˈdja.blos/

Examples
La película trata sobre una lucha entre ángeles y diablos.
The movie is about a fight between angels and devils.
En el carnaval, muchas personas se disfrazan de diablos.
At the carnival, many people dress up as devils.
Según la leyenda, los diablos viven en el infierno.
According to the legend, devils live in hell.
Plural of 'diablo'
This is simply the plural form of 'diablo' (devil). Like most Spanish nouns ending in a vowel, you just add an '-s' to make it plural.
monstruos
mohn-STROH-ohs/ˈmons.tɾwos/

Examples
Esos criminales son unos monstruos, no tienen corazón.
Those criminals are fiends; they have no heart.
Los monstruos de la guerra causaron un daño irreparable.
The monsters of war caused irreparable damage.
Figurative Use
This meaning uses the idea of a physical monster to describe someone's terrible character or actions. It's a very strong insult.
Diablos vs. Monstruos
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