Definition of fiends:
(n) :
A very evil person.
(n) :
(informal) An addict or fanatic.
(n) :
A devil or demon; a malignant or diabolical being; an evil spirit.
(v) :
(slang, intransitive) To yearn; to be desperate. [(often) with for ‘something, especially drugs’]
(n) :
(religious, archaic) The enemy of mankind, specifically, the Devil; Satan.
(n) :
(obsolete) An enemy; a foe.