I don't know the context that you are asking this question in, so I can't connect it to whatever book you are reading or anything. But here's the general idea of the term.
"Fiend" of course, is a word meaning something like a devil or a demon. "Intemperance" means not doing things in moderation -- going overboard when you do stuff. More specifically, it is the opposite of "temperance" -- temperance is usually defined as not drinking any alcohol.
So, intemperance means drinking alcohol, possibly too much alcohol. Fiend means devil. So the two words put together mean something like "demon Rum" -- the idea that drinking alcohol is like a demon that will ruin a person's life.
I hope that makes sense in context of whatever it is you are reading...