The Celestis were an amoral, petty self-interested group who abandoned the Great Houses shortly before the War and saw themselves as aloof gods far above the concerns of the Spiral Politic. (PROSE: The Celestis)
The Celestis began as the Celestial Intervention Agency, (PROSE: Alien Bodies) a prominent interventionist group among the Great Houses. They took the idea of the Houses as gods of history to extremes, violating the Protocols to commit acts of retroactive genocide at whim. (PROSE: The Celestis)
When word of the coming War first reached the Homeworld, the interventionists were the only ones to immediately grasp the full implications. In the traditional conception of war, the aim of one's enemy was destruction, and even if one lost one could die in the thought of a life well-lived. But in War on a this literally historical scale, losing wouldn't mean death: losing would mean that one had never existed in the first place. In the warped logic of the Celestis, the solution was obvious: beat the enemy to the punch. The enemy couldn't erase their history if they'd already erased it themselves.
And so the Celestis very carefully excised themselves from the past in such a way that although their physical bodies no longer (ever had) existed, the idea of them lived on, and their realm — Mictlan — was founded. On the Homeworld the Celestis were remembered as traitors who had abandoned the Houses in their hour of greatest need.
The Lords Celestial were fractious and divided to the point that each Lord (the title was used regardless of gender) was considered a House: a total of "ninety-nine Houses," although they also sometimes referred to themselves collectively as the "Celestial House." They presented no united front in the War, and individual Lords often interfered on behalf of one side or another as if it were all a game. The conceptual entities such as anarchitects and Shifts, which they developed from the same flux theory used to found Mictlan, were supplied to the Great Houses' enemy.
As conceptual entities, the Celestis required real people to believe in and think of them if they were to continue existing. To help achieve this, they each had a wardrobe of god-forms used to intimidate the lesser species. Their standard mode of operation was to appear as a relevant deity and offer a Faustian bargain, offering to extend a subject's life if they would receive the Mark of Indenture and come to Mictlan after their death. (PROSE: The Celestis, Lords Celestial)
The Celestis created Amerika through a compact with Thomas Jefferson's witch lodge, and became Amerika's Gods. In conjunction with Faction Paradox, they eventually betrayed the nation and erased significant parts of it from history. (PROSE: Print the Legend)
The Celestis noticed that their own human agents were all resurrected in the City of the Saved after the Celestis had recorporated them, which implied that something would eventually happen to destroy Mictlan. These ex-slaves established the Ghetto of the Damned to oppose the Celestis. As part of an ambassador-exchange program with the City, the Celestis sent their Lord Foaming Sky to the City. He created an army of duplicates of himself with worldofme devices and tried to conquer a City District so that the members of the Ghetto could be forcibly shipped back to Mictlan and interrogated, but the Rump Parliament disposed of all of the duplicates and he returned to Mictlan in disgrace. (PROSE: Amanda Legend Lefcourt, Lord Foaming Sky)