Celestis

The Celestis were the Celestial Intervention Agency after they removed themselves from history to escape the War in Heaven, (PROSE: Alien Bodies) becoming an amoral and selfish group of conceptual entities who saw themselves as aloof gods far above the Spiral Politic. (PROSE: The Book of the War)

Origin
The Celestis began as the Celestial Intervention Agency, (PROSE: Alien Bodies) a prominent interventionist group. They believed that the Great Houses were gods of history, which allowed them to casually violate the Protocols and commit acts of retroactive genocide at whim. When word of the coming war first reached the Homeworld, the interventionists understood that, unlike in a regular war, where one simply risked losing their life, in a time war the risks involved losing one's history.

Rather than being forcibly removed from time by the enemy, the Celestis decided to exit history on their own terms, so they carefully excised themselves from the past in such a way that, though none of them had ever physically existed, their idea survived. They were remembered on the Homeworld as traitors who abandoned the Houses in their hour of greatest need.

The Celestis, now conceptual entities rather than physical beings, used flux theory (PROSE: The Book of the War) and the Matrix to build themselves a world outside spacetime called Mictlan. (PROSE: Alien Bodies)

Though they sometimes referred to themselves collectively as the "Celestial House", the Lords Celestial were fractious and divided to the point that each Lord was considered their own House, making a total of "ninety-nine Houses". (PROSE: The Book of the War) They were split on whether to support the Time Lords or their enemy in the War in Heaven, (PROSE: Alien Bodies) and individual Lords often interfered on behalf of one side or another as if it were all a game. Conceptual entities made by the Celestis, (PROSE: The Book of the War) such as anarchitects and Shifts, were supplied to the enemy. (PROSE: Alien Bodies, The Book of the War)

Interactions with humanity
Though they were much more difficult to destroy in their new existence as ideas, (PROSE: Alien Bodies) the Celestis required real people not only to act on their behalf but also to think of them in order for them to exist. To achieve this, they often appeared to lesser beings in a "god-form" in order to intimidate them, then offering a Faustian bargain to extend their life in exchange for the Mark of Indenture. (PROSE: The Book of the War) Bearers of the Mark would then serve in Mictlan for eternity after their death. (PROSE: Alien Bodies)

The Sidhe were aware that the Celestis were collecting those that died during the Battle of the Bulge. (PROSE: Autumn Mist)

The Celestis created an "agency" of Investigators to act on their behalf in the inner universe, generally as detectives and assassins. (PROSE: The Book of the War)

The Celestis sent a recorporated agent called Trask to represent them at the auction for the Relic. (PROSE: Alien Bodies)

The Celestis made a deal with the early 21st century scientific genius Jimmy Moriarty which involved giving him knowledge of a ritualistic method of time travel. (PROSE: Erasing Sherlock)

By making a compact with Thomas Jefferson's witch lodge, the Celestis created Amerika and became its Gods; together with Faction Paradox, they eventually betrayed the nation and erased significant parts of it from history. (PROSE: Print the Legend)

The Celestis established complete control over the government of a major country on early 21st century Earth. (PROSE: Head of State)

Destruction
Because the City of the Saved included many humans with memories of servitude in Mictlan, the Celestis knew that their realm would one day be destroyed. They sent Lord Foaming Sky to the City through an ambassador exchange program, and there he used worldofme devices to create an army of duplicates of himself. He tried to use these duplicates to conquer a City District, in the hopes that this would give him the leverage to force ex-slaves (organised into the Ghetto of the Damned) back to Mictlan, where they would be interrogated. However, the Rump Parliament intervened in the attack and destroyed all the duplicates. Foaming Sky returned to Mictlan a disgrace. (PROSE: The Book of the War)

After the Eighth Doctor put a Celestis Mark of Indenture on the Shift, it was kept in a zoo in Mictlan and lived through the fall of the Celestis under the Memeovore. (PROSE: The Book of the War) This came to pass when their former agent, One, discovered that Mictlan could lure Swimmers to the universe; to stop the universe from being devoured in this way, he arranged for Mictlan to be devoured by the Memeovore, and the Eighth Doctor and a fleet of War TARDISes cut the realm away from the rest of the universe. As a result, the Celestis were virtually all destroyed. (PROSE: The Taking of Planet 5)

Behind the scenes

 * 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 in becoming the Celestis. (TV: The End of Time)