Also known as Ringwraiths, Black Riders, or just the Nine, the Nazgûl are what you get when you give nine perfectly ordinary mortal kings each a ring of overpowering evil. Tend to dress in black robes, screech a lot, and be extremely hard to kill, what with being dead already. Taking on a Ringwraith is a Very Bad Idea, unless you want be killed or turned into a lesser wraith by getting stabbed in the heart with a Morgul-knife. That would be a Fate Worse Than Death.
In Suefic, Sues are sometimes daughters of Ringwraiths, notably the Witch-king of Angmar, who is the leader of the Nine. Agent Diocletian was one such before she was reformed. It's also not unknown for badficcers to create an Extra Ringwraith.