Akhaten was a parasitic, sentient planet that fed on the souls, and therefore the stories, of the Sun Singers of Akhet. He was considered by the people of the neighbouring worlds to be a great and fearful god. Within his rings was the asteroid Tiaanamat, in which was held the Festival of Offerings, which served to keep him asleep.
Biography
It slumbered for thousands of years until the Long Song ended, when he 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 the Old God '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 overload of stories caused him to implode. (TV: The Rings of Akhaten)