The Ringwraiths, also known as the Nazgûl, are a group of fictional creatures seen in J.R.R. Tolkien's Works.
The Ringwraiths were originally nine human kings, whom the demonic Dark Lord Sauron tempted with the power of nine bewitched Rings that, bound to Sauron's very soul, would give them access to some of his powers. The Rings did do this, but actually also bound them to Sauron's will, making them his slaves and heralds. They also extended the men's lifespan, but only at the cost of turning them into undead wraithlike beings incapable of feeling goodness and invisible to any not connected to the power of Sauron themselves.
In this form, they served as Sauron's elite soldiers, and, after Sauron's disappearance, were greatly reduced in power and sought out the master-ring to which Sauron's soul was bound, which would allow them to return their master to his former glory. Their leader was more powerful than the others and was known as the Witch-king of Angmar.