Template:Infobox Organisation The Celestis (or House Celestial) were the Celestial Intervention Agency after they had removed themselves from the universe and became conceptual entities to preemptively escape temporal damage in the Second War in Heaven.
The Celestis removed themselves from history and detached themselves from the physical plane of existence, making themselves ideas, and therefore much harder to destroy than a physical body (which is tied down to a timeline). They inhabited a world outside the physical universe called Mictlan, a place constructed using the Matrix. During the War, they were split on whether to support the Enemy or the Great Houses of the Time Lords. (PROSE: Alien Bodies) They supplied help and technology to both sides.
Since they existed solely as a network of ideas, their survival depended on minds to believe in them, so Lords of the Celestis would frequently manifest to lesser species as gods and demons. (PROSE: The Book of the War) One common trick was to take those on the brink of death to Mictlan. (PROSE: Autumn Mist) There, they would be offered an extended lifespan in return for eternal servitude in Mictlan after their eventual death.
Besides, the Lords and Ladies of the Celestis rarely left Mictlan, usually acting in the outside universe through their fearsome proxies, the Investigators. (PROSE: The Book of the War) However, many lesser races like the Sidhe were aware of the Celestis. (PROSE: Autumn Mist)
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).