The Celestis removed themselves from history and detached themselves from the physical plane of existence, making themselves ideas, and so harder to destroy (harder than a physical body tied down to the timeline).
They believed themselves gods and aloof from the Spiral Politic. Due to the nature of their existence, they required people to act on their behalf as well as think of them to exist. To do this they often appeared to lesser beings in the form of gods. (PROSE: The Book of the War)
The Celestis were virtually all destroyed when their former agent, One, arranged for the Memeovore to devour Mictlan and be subsequently cut away from the rest of the Universe by the Doctor and a fleet of War TARDISes, when he learned that Mictlan could attract the vast Swimmers to this universe (PROSE: The Taking of Planet 5).
Behind the scenes Edit
Rassilon's plan for the Time Lords to survive the Ultimate Sanction at the end of the Last Great Time War by becoming "creatures of pure consciousness" resembles the actions taken by the CIA during the Second War in Heaven in becoming the Celestis. (TV: The End of Time)