One (The Taking of Planet 5)

Investigator One was the Celestis' head field agent. (PROSE: The Book of the War)

The Celestis made him from the germ-plasma of their slave class as part of an experiment. He was raised in the doom-creches of Mictlan's Baby Farm, surrounded by ornamental skulls and faceless wet nurses.

After serving the Celestis for a considerable time, One was influenced by the hermit into going rogue. While sent to clean up the Celesti operation in the Seventh Retro War, One intentionally went against his orders and let some pockets of resistance survive. Because of this, Two was assigned to observe One.

The Celestis sent One and Two to find out what had caused a Lord Celestial to not only cease to exist, but to have never existed.

In fact, One was responsible for the non-existence of the Lord Celestial; he was working with the hermit to destroy Mictlan before it would attract the attention of the Swimmers and cause the universe to be destroyed. To that end they arranged for the Fendahl Predator to be released from its time loop prison. Since it fed on concept, and Mictlan was a conceptual construct, the Predator could not resist feeding on it. The inhabitant whose existence it first erased was the one that caused One and Two to be sent to investigate in the first place.

After Mictlan was excised, One met up with the hermit on Earth. He had adopted the form of "an inconspicuous man in a grey suit", but due to "damage caused by sustained staser fire from Xenaria's troops", he was no longer able to fully maintain a physical form: his features were "blurred, like an unfocused photograph of a man". His "brush with death" had left him "sobered, less boastful than before", and he seemed to resent the Time Lords. The hermit asked One to shoot him in the heart to trigger a regeneration so that he could abandon his monstrous form for a more inconspicuous one. One and the Hermit then parted ways. (PROSE: The Taking of Planet 5)

The Rivera Manuscript described a Great House renegade's imprisonment by mysterious forces believed to be representatives of the Enemy. The renegade's captors were never directly described, but the manuscript referred to a humanoid form, a non-carbon presence, and something the renegade called "the first, the many, and the indivisible" which "may have been a bad translation"; in parts of the renegade's subsequent praxis-induced vision of the Event, he was accompanied by someone called "One". The Book of the War 's commentary noted the shared name with the head Investigator of the Celestis, but did not know what to make of it, as the Celestis themselves did not appear to be involved.

Though there seemed to be difficulty translating the relevant section of the Manuscript into English, some sections of the dialogue between this "One" and the renegade legible: an interpolation reading "why does it hate us?" and, later, an extended conversation. (PROSE: The Book of the War) "Is that what you're planning / It might be / It still could be / Remember / Inside the skin of the sun / It's an option / It wouldn't work / What if someone tried it / What if you were called to account / It's an option / Always an option / You keep the sun / In a bottle / You want to know what happens if you lose control / Don't you?"

- The Rivera Manuscript

Shortly before her death, Thessalia sent an unprecedented order directly to the ruling Houses, insisting they perform a thorough spectroscopic analysis on the Homeworld's sun.

Notably, other references in the Book of the War were made to the phrase "One" as something of much greater and more incomprehensible significance than a mere representative, including the vision Umbaste experienced within the caldera and allusions Compassion made while speaking to an advisor of the War King. This usage of the term "One" seemed to get at the very nature of the Enemy. (PROSE: The Book of the War)