They were first appeared in the Second Age, and were soon known as Sauron’s principal servants. They became more known and explicitly mentioned in the Third Age during the War of the Ring. They were eventually all destroyed at the end of the War of the Ring.
They were difficult to harm, as they existed mostly in the Unseen (in Arda). This means that weapons will not harm them. But they were not able to interact with the Seen, or what they call the Ordinary World.
The Nazgûl that were identified were:
- Witch-king of Angmar (leader)
- Khamûl (second in command)
- 3 Númenoreans