In The Lord of the Rings: The Return of the King, Amon Dîn is the beacon mountain that begins Gondor’s warning signal to Rohan.
Amon Dîn is one of the beacon hills in Gondor. When Minas Tirith is threatened, the first beacon is lit there, starting a chain of fires across the mountains. The signal eventually reaches Edoras, where Théoden learns that Gondor needs military aid.
The film compresses and dramatizes the beacon system, presenting the fires as a spectacular line across the landscape. Amon Dîn is therefore the first named mountain in the sequence, not the city of Minas Tirith and not Mount Doom, where the Ring is destroyed. The beacon chain is a communication system rather than a magical event: each watch post observes the previous fire and lights its own. This moment helps trigger Rohan’s ride to Gondor’s aid.