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While the content of this article is based on official information, the actual name of the subject is conjectural, and is yet to or cannot be officially named.
The creature belonged to a group of ancient beasts, discovered by Sauron and fed with foul meats. It was the largest of the nine winged steeds that were given to the Nazgûl after they were unhorsed at the Ford of Bruinen.
Although mainly riding upon a horse for the majority of the trilogy, the Witch-king took up the fellbeast towards the end of the War of the Ring. When the Rohirrim arrived at the Fields of Pelennor, the Witch-king quickly rode off and mounted the fellbeast. In battle, he shot King Théoden's horse, which fell and pinned the king to the ground. Landing on the fallen king, the fellbeast dug its claws into the flesh of Snowmane. Dernhelm (who revealed herself as Éowyn) defended the dying Théoden, and challenged and beheaded the beast.
Even headless, the beast yet lived a few seconds before it died. Its master, the Witch-king, furiously attacked Éowyn, and almost killed her, but she survived and eventually killed him with the help of the hobbit Merry.
Its headless corpse was later burned, and left a black, bare stain on the ground where it lay. Not more than ten days later, Sauron himself would be destroyed, and all the remaining fellbeasts would perish in the ruin of Mordor, along with their unearthly masters.
Portrayal in adaptations
The Lord of the Rings film trilogy
In The Two Towers movie, in the fellbeast's first appearance, it flies over Osgiliath as it is being attacked by Mordor, and it almost captures the hobbit Frodo Baggins after the Witch-king senses that the One Ring was with the halfling. But before the fellbeast attacks Frodo, the other hobbit Samwise Gamgee saves him, and Faramir of Gondor shoots an arrow at the fellbeast's neck. Frightened, the creature flies away to Mordor.
It was especially prominent during the Battle of the Pelennor Fields, where it, along with the other fellbeasts, slaughtered many Gondorian soldiers, grabbing them and their horses with its powerful claws, lifting them high into the air, and dropping them to the ground. It, unlike the other fellbeasts, wears a helmet, seemingly to show that it has superiority over the other flying monsters. Later, when the Rohan army arrived to help Gondor, the Witch-king commanded the fellbeast to attack King Théoden of Rohan, and feast on his flesh.
The fellbeast attacked and quickly killed Snowmane, Théoden's horse. The horse fell on Théoden, crushing him. He eventually dies of his wounds but not before he sees his niece kill the Witch-king. As in the book, the fellbeast attempts to kill Eowyn, but she swiftly darts aside and cuts the beast's neck with two strokes. The headless corpse continues to writhe as it dies, flinging down the Witch-King from his saddle.