Ringwraiths

Nazgul or the Nine are the Numenorean Princes and other lords of Middle-earth who were given nine Rings of Power by Sauron. They proved the easiest to bend to his will. They lived for centuries until they became shadows, enslaved by Sauron and becoming his most powerful servants. Their greatest was the Witch-King who ruled Angmar. He cannot be killed by a mortal man.

They use corruptive dark magic to enance their powers. They have magicians as their apprentices. They are invisible and for this they wear black robes, and use fear and sorcery. They ride black steeds or swift fearsome winged beasts.

Even when slain, they usually abandon their forms and flee as shadows back to Mordor to take new shapes.