The Nazgûl, also known as the Ringwraiths, are fictional characters in J.R.R. Tolkien's 'The Lord of the Rings'. They were once mortal kings, men who were given Rings of Power and became servants to Sauron, the Dark Lord. Shrouded in darkness, they are terrifying figures who pursue the One Ring to return it to their master. The Nazgûl are known for their formidable presence and their relentless pursuit of Frodo Baggins, the series' protagonist. They play a crucial role in demonstrating the power and reach of Sauron throughout the series.