Ringwraiths or Nazgul as they are known in the black language of the Orcs are the mightiest and most powerful of all Sauron the Ring Lord's evil Servants. They emerged in the twenty-third century of the Second age of the Sun in Middle-earth.
The Nazgul were once powerful kings and Sorcerers among men who wore Rings of Power given to them by Sauron. These were the nine of the Nineteen Rings of power forged by Celebrimor and the Elven Smiths of Eregion for Sauron. For many years these rings were used by their bearers to fulfil their worldly desires. All of these rings were ruled by the One Ring of Sauron. By the twenty-third century, these men had lived so long by their rings that they had been transformed into wraiths who thought only of serving their master, Sauron. The wraiths wandered Middle-earth committing terrible deeds in the name of Sauron, the Ring Lord.
The Nazgul were terrible in apearance. They wore great black cloaks with hoods and beneath, mail shirts and helms, their bodies were invisible and decayed and any who looked into the darkness within their hoods, were horrified. Sometimes in the darkness where their faces should appear, two glowing , hypnotic eyes could be seen.
Numerous weapons were used by the Nazgul, swords of Steel and Flame, poisoned daggers and maces. The blades of the Nazgul were forged in the land of Mordor and a wound from such a weapon would turn a creatures heart cold and it would become a servant of Sauron. The Nazgul were untouchable to mortal men. Blades could not harm them unless magically forged by the elves and any blade that touched them would perish.
The Tolkien Encyclopedia
The Encyclopedia of Middle-earth
Hypertextual Systems by FMI Publishing, 1996