Nazgûl

What you get when you give a perfectly ordinary mortal king 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.

In Suefic, Sues are sometimes Daughters of Ringwraiths, notably the Witch-King of Angmar.