Akhaten was a parasitic, sentient planet that fed on the souls of the Sun-singers of Akhet. He was considered by the people of the neighbouring worlds to be a great and fearsome god. Within his rings was the asteroid Tiaanamat, on which was held the Festival of Offerings, which served to keep him asleep.
Akhaten slumbered for thousands of years, leaving a Mummy in a pyramid as an alarm clock, until the Long Song ended. Akhaten then awoke and threatened to consume all seven of its neighbouring planets. Eventually, the Eleventh Doctor let him take his memories instead of the valuable things of the natives of Akhet. The Doctor was then saved by Clara Oswald, who gave Akhaten 'the most important leaf in human history', the leaf that told all of her family's story, all that happened, all that didn't happen and all that should have happened. The near-infinite possibilities of causality caused Akhaten to implode. (TV: The Rings of Akhaten)