Definitions for "Fiend"
An implacable or malicious foe; one who is diabolically wicked or cruel; an infernal being; -- applied specifically to the devil or a demon.
a cruel wicked and inhuman person
one of the evil spirits of traditional Jewish and Christian belief
Primarily refers to baatezu, gehreleths, hordlings, tanar'ri, and yugoloths, but sometimes includes any intelligent being native to the Lower Planes (night hags, imps, quasits, and the like).
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"--Winston Churchill