Long Song

The "Long Song" was, in the words of the Eleventh Doctor, a "lullaby without end" sung to feed the Old God and keep him asleep. By the time of its ending, it had been going on for millions of years, Chorister handing it to Chorister, from generation after generation.

It was sung by the Queen of Years along with the Chorister at the Festival of Offerings, where all the species from the local system would gather, and offer valuable things to the Old God.

The Eleventh Doctor was familiar with the song, although he had trouble keeping up with the chorus in practice.

When the Eleventh Doctor and Clara Oswald visited the planet to watch the Festival, the song ended with the Chorister, and both the Mummy and the Old God woke up. The Chorister kept on singing, trying to soothe them back to sleep. When the Queen and the Chorister were captured, the Chorister teleported out, and the Doctor saved the Queen. (TV: The Rings of Akhaten)

Behind the scenes
Interestingly enough, the series 7 soundtrack lists the music sang by the Chorister and Merry during the Doctor's speech as "The Long Song", while the song definitively referred to in the episode as the Long Song is entitled "God of Akhaten".