31 Monsters of October, Day 1: Nazgûl (Ringwraiths)

Real World Origin:
Literature, The Lord of the Rings by J.R.R. Tolkein

In-Universe Description:

In J. R. R. Tolkien's The Lord of the Rings the Nazgûl are servants of the Dark Lord Sauron. After Sauron forged several "Rings of Power", he distributed them to various individuals (three for the elves, seven for the dwarves, and nine for men). Of these rings, the nine given to men held the greatest power over their wearers, eventually making them completely subject to the "One Ring" held by Sauron. As described in Tolkien's The Silmarillion:

Those who used the Nine Rings became mighty in their day, kings, sorcerers, and warriors of old. They obtained glory and great wealth, yet it turned to their undoing. They had, as it seemed, unending life, yet life became unendurable to them. They could walk, if they would, unseen by all eyes in this world beneath the sun, and they could see things in worlds invisible to mortal men; but too often they beheld only the phantoms and delusions of Sauron. And one by one, sooner or later, according to their native strength and to the good or evil of their wills in the beginning, they fell under the thraldom of the ring that they bore and of the domination of the One which was Sauron's. And they became forever invisible save to him that wore the Ruling Ring, and they entered into the realm of shadows. The Nazgûl were they, the Ringwraiths, the Ulairi, the Enemy's most terrible servants; darkness went with them, and they cried with the voices of death.

As described above, the nine became invisible to all but the wearer of the One Ring. All others see only the cloaks and armor they wear. The chief of the Ringwraiths, who served Sauron for over 4,000 years, is known simply as the Witch-king of Angmar.