fiend fiend /ˈfiːnd/

noun

1. () A cruel wicked and inhuman person  ( demon , devil , monster , fiend , ogre )
   a cruel wicked and inhuman person
     The notorious criminal was known as a fiend for his ruthless acts.
   The notorious criminal was known as a fiend for his ruthless acts.
   
2. () An evil supernatural being  ( daemon , daimon , fiend , demon , devil )
   an evil supernatural being
     According to folklore, the demon haunted the abandoned mansion on the hill.
   According to folklore, the demon haunted the abandoned mansion on the hill.
   
   
3. A person motivated by irrational enthusiasm (as for a cause)  ( fiend , fanatic )
   a person motivated by irrational enthusiasm (as for a cause)
    A fanatic is one who can't change his mind and won't change the subject
   A fanatic is one who can't change his mind and won't change the subject