Who were the Nazgul?

[Legate of Amon Lanc]

Yes, I think TM quite got it. There is not much information about them apart from what was said here. Only to that fire thing,

Sauron can put fire to his evil uses, as he can all things, but these Riders do not love it, and fear those who wield it.

I think fire might represent here the cleansing power, something which gives light in the darkness, and also the heat: as an opposite to cold and death the Riders represent. It is just a speculation, however, can't think of much more.