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
If you describe someone as a fiend, you mean that they are extremely wicked or cruel. A tearful husband repeated calls for help in catching the fiend who battered his wife. = monster
Fiend can be used after a noun to refer to a person who is very interested in the thing mentioned, and enjoys having a lot of it or doing it often. if you're a heavy coffee drinker or strong-tea fiend