Nazgûl

The Nazgûl, Ringwraiths or Úlairi (also called the Black Riders, the Nine Riders, or simply the Nine) were servants of Sauron, first appearing in The Lord of the Rings. They originally were human and leaders of Men in the Second Age. Sauron gave to them nine of the Rings of Power, which brought them under his will. Over time they became like undead ghosts, invisible to the normal world. The greatest weapon of these horsemen was not swords but sorcery and fear.

After annoying Farmer Maggot, terrifying Sam's father, and nearly killing Fatty Bolger, some of them pursued Frodo and his three hobbit friends to the Brandywine, where they barely escaped by ferry. Several attacked Strider and the four hobbits in Bree and at Weathertop. In a dramatic confrontation at the ford of Rivendell, they were unseated and apparently drowned by Gandalf. However, only their horses drowned, and Sauron gave them new steeds, flying monsters with great leathery wings.