Eighteen was known as one of the most eccentric Investigators. He was often dispatched to escort servants from the outside universe to Mictlan upon their deaths, especially if they came from worlds in Warzones or monitored by the Great Houses. Eighteen would amuse himself by putting these new servants on trial, showing them the entire Spiral Politic and forcing them to defend their right to exist to a jury of their world's most famous dead. However, the court and its jury were always just a sham created by Eighteen, and the servant would become a slave in Mictlan no matter the result. (PROSE: The Book of the War)
The Celestis elevated Eighteen to single digit status when they sent it on a mission with Investigator One. On the surface, One and Two were sent to the wider universe to find out what had caused a Celesti to not only cease to exist, but to have never existed. During that investigation, Two was to watch One, whom the Celestis feared had gone rogue; if this was confirmed, Two was authorised to eliminate One. Upon discovering Two's true purpose, One attacked her and bashed her skull into a pulp; however, the nanites in Two repaired the damage, reconstructing its memories so it believed its mission was to kill all humanoids.
Two eventually regained enough memory to remember Mictlan and TARDISes, and it developed a urge to return home. Upon finding Hume, it decided to eat his brain and absorb his knowledge so it could build at least a rudimentary TARDIS and return to Mictlan. While Hume and Two fought, Fitz Kreiner was able to finish repairing the Celesti fictional generator and he used it to turn Two into a work of fiction: a 1930s-looking magazine called Spicy Archon Stories. (PROSE: The Taking of Planet 5)