But it had loopholes, Immortals could live alongside humans, could assimilate, interbreed with them, among other things.
This deal would be called the Declaration of Saud, named after the Catakon God of Fate.
The former were intrested in assimilating with humanity, bringing their tecnology to help the humans ascend, the latter wanted to leave humanity alone to their devices. They agreed to a deal, they will peer into the future for the exchange of not being able to deviate the timeline. They were unable to prevent the fall of the Roman Empire, the endless wars of the EUropeans, and the disastrous future of the Africans and Americans in chains.
They peered into the realm of souls and found the first congengrations of daemons, feeding on the sins of humanity, and somehow on the Immortals. That is when they started using the Immaterium to their advantage once more, like the Old Ones that their souls were descended from. They weathered through conflict and atrocity, becoming two factions of the Immortals: The Assimilas, and The Presevas.
They were basically the ancient underground civilizations of the lower crust, ignorant of the surface, until the beginning of A.D, when explorations were conducted and assimilation began. The Immortals learned the languages of the world, hiding their technology and posing as the nobility of made-up countries, or so some thought. That is when the Immortals saw the sins of mankind. Why couldn't they just be as pure in the outside as in the outside?
The Strike Empire was founded on the quest for knowledge, for the Immortals, a sect of Old Ones that bound their souls into humans. These humans were powerul psykers, but ones that never ventured above the surface until approx. 1 A.D, hidden from most of history. These communities created large cities of warp-stone and steel, their weapons extremely durable and flexible, their armor easily shiftable...
Chapter 1 = The Federation of the Light: A Chaotic Order