While the Hobbits are staying in the Prancing Pony at Bree, their room is rummaged by several Black Riders. Aragorn then explains that the Black Riders are the Nazgûl. The Nazgûl were once nine kings of Men. They were given nine rings of power by Sauron and were corrupted. They were then doomed to become wraiths and still serve Sauron. They had been sent to find The Shire and Baggins after Gollum had revealed those details.
Arwen then is pursued by them along the East Road as she carries Frodo to Rivendell. She reaches safety at the Ford of Bruinen just ahead of the Black Riders by bringing the river down on them, which drowns their steeds and sweeps them away.
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