Director Peter Jackson approached Enya, asking if she would be interested in writing a song for The Lord of the Rings. Thrilled at the prospect, Enya headed to New Zealand to see the preliminary edits of the film.

Enya worked on the song with Nicky Ryan, her producer, and Roma Ryan, her lyricist. Nicky produced Enya's vocals and arranged the music while Roma wrote the lyrics. They recorded the song through Enya's contract with Warner Music in the Ryans' Dublin studio, Aigle Studio. The vocals were recorded in "Aigle Studios", Enya's Studio near Dublin and the orchestration was recorded in London, directed by Howard Shore and performed by the London Philharmonic Orchestra. Compositionally, the piece is simple, featuring a backdrop of choir and strings.

“I wanted Enya’s voice,” says Shore. “She wrote and I orchestrated, so it’s a seamless sound. Her singing grows right out of the choral music and the orchestra.”

The lyrics of this theme song include English words, as well as words in the fictional Elvish language, Quenya, created by J.R.R. Tolkien. Tolkien began writing this High-Elven language in 1915 at the age of 23. Quenya went through many revisions before it finally appeared in the books. Tolkien's son Christopher said that for his father Quenya was "language as he wanted it, the language of his heart." Tolkien himself said of Quenya, "Actually it might be said to be composed on a Latin basis with two other ingredients that happen to give me 'phonaesthetic' pleasure: Finnish and Greek. It is however less consonantal than any of the three. This language is High-elven or in its own terms Quenya."

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Wikipedia