The hidden Elvish city in a valley in J.R.R. Tolkien’s The Lord of the Rings is Rivendell.
Rivendell is also known by the Elvish name Imladris. Elrond establishes it in the Second Age as a refuge for Elves and their allies. In the Third Age, the settlement becomes an important center of learning, memory, and resistance to Sauron. Its secluded position in a valley helps protect it from hostile forces.
Frodo reaches Rivendell after being wounded by the Witch-king on Weathertop. Elrond heals him, and a council later decides that the One Ring cannot remain in Rivendell. The members of the Fellowship of the Ring then depart from the city to continue the quest.
Rivendell is distinct from Lothlórien, another Elvish realm ruled by Galadriel and Celeborn. Gondolin is a hidden city from the First Age, while Eregion was the Second Age land where the Rings of Power were made with Sauron’s influence.