!Fluff warning! On Ultima Tectum campaign:
Noospheric Space was necessary to develop during the Dark Age of Technology to contain algorithms of the final stage of AIs.
Turning of the Golden Age of Technology to Dark Age of Technology can be pointed to the moment, when algorithms of the final stage AIs were involuntary extracted from C’tan of Mars.
Greatest scientists of Solar Hegemony were able to create the group of templates from C’tan extraction and Novabyte code were implemented upon those templates. Thus Solar Hegemony creates the group of Final stage AIs which hijack humanity until the Age of Strife.
Final stage AIs of Dark Age of technology existed only in Noospheric Space and required lesser AIs to interact with tasks of the Materium.
Psychic awakening of the humanity, engineered by the old ones, starts to warp final stage AIs and in response they initiate galaxy wide civil war.
AIs are divided between four factions: Corrupted, Terminators (exterminators of the humanity), Asimovers (protectors of the humanity) and Impartial.
At the beginning of the Age of Strife Iron Wars wipes final stage AIs to oblivion in internal conflict and after milleania, at the age of the Imperium some alleged contacts with lesser AIs has been recorded but not proven.
Lesser AIs were not as prone to psychic background noise of awakening psykers of the humanity and were able to shield themselves with Hexamathic Code and Aegis Barrier. Legends of UR-025 and Starfighter-31 haunt Imperial records as voids of no information.
Omnissiah was one of the final stage AIs before collapse of the Golden Age of technology. It belonged to Asimover faction. It’s dominion was over Mars and Cult Mechanicus are it’s hierarchy of servants, which continue to fulfill it’s orders millenia after it has perished.