The 23rd millennium.
Humanity has reached the apex. Spanning of over 500,000 worlds scattered all across the galaxy.
Their Golden Age. Prosperity was observed in all ranks of the society. From the lowest rungs to the highest of authority. Baseline human augments were the norm.
State-of-the-art facilities, technologies, and buildings were used by everyone.
Psykers were first scientifically verified to exist at some point towards the end of the 22nd Millennium, making a sudden appearance on almost every Human world within a relatively short span of time. It was only then that psykers were a confirmed phenomenon, after long millennia of legend, rumor, and pseudoscience in which the scientific establishment simply could not prove the existence of psychic powers among Humans.
On the more progressive and technologically advanced Human colonies, these psykers were protected by law and allowed to develop and explore their abilities. On other, less sophisticated Human worlds, they were often killed in literal witch-hunts and anti-psyker programs.
At this time, with the help of the Warp-Drive, which allowed interstellar transportation, commerce, and communications to develop, the Human colonies were politically united with Terra in some form of federated interstellar government, thus allowing the Human species to remain unified and powerful.
Casualties from any forms of sickness, old age, and battle were reduced to almost null. Thanks to the combined efforts of humanity's most brilliant minds and their powerful creations, various forms of Artificial Intelligences.
The Men of Gold, a mysterious sub-sect of humanity, is known to be both physically and emotionally superior to baseline humans. Appeared sometime during the Age of Terra, when humans were still within the confines of their own system. Watching and learning from Mankind as they establish Order and Civilization in the anarchy of nature. They created the Men of Stone with the purpose of colonizing the Galaxy.
The Men of Glass, the first generation 'simple' machine intelligences only available in the interfaces of Mankind's communication and administration systems. Fading away in obscurity as they were overshadowed following the creation of the Men of Stone.
The Men of Stone, created during the 21st millennium, was engineered by the Men of Gold for the purpose of deep space colonization. Capable of building great artifices and constructs.
The Men of Iron, created by the end of the 21st millennium, successor to the 'Men of Stone'. Artificially intelligent humanoid thinking machines, known to be loyal only to Mankind. Served as the main army and labor force of humanity for the following millennia.
Their combined armies, unmatched.
Systems held by humanity were protected by multitudes of superstructures, system-covering defensive grids, and incredible defensive fleets.
Huge battlefleets regularly patrol the borders to counteract anyone foolish to challenge humanity's might.
Their fleet compositions were so strong that the Orks themselves were seen as just a 'minor inconvenience' or 'pests' and a past-time target practice for their ships' weapon systems.
The Aeldari, even in their arrogance, refused to face humanity at its peak and resolved to turtle themselves in their mysterious web ways and core worlds. Working through schemes in the shadow of their former brilliance and in the face of a 'pacifistic' isolationist in diplomacy.
Necron incursions in some worlds were contained and hidden in classified secrets as humanity's most forefront scientists study such Xenos. Humanity's armies were more than a match against such techno Xenos.
Pacifistic aliens (or Xenos) enjoy the benefits through trade and Industry in the form of a galaxy-spanning Federation of sentient species with humanity at its lead, a hegemony to be precise.
Everything was on the onset of prosperity enjoyed by countless sentient species, that would have lasted for several thousands of millennia yet the Dark Forces stirring in the shadows won't let that happen.
Various small factions of the Aeldari, still bitter from their 'fall' from power in contrast to what humanity had, could never accept that these 'Mon-keighs' become the hegemon of the galaxy as they once were. They colluded with rogue Chaotic elements for the creation of Humanity's greatest nightmare, the Ruin-Code.
The Ruin-Code, a bio-mechanical virus that is perhaps one of the most successful and potent weapons that the Ruinous Powers ever made. It corrupts technology into doing Chaos' bidding. An almagation of corruption, desire, death, and wrath. To ruin technology caches and databases into corrupted tech.
Spreading into the various spanning networks of mankind, nigh unstoppable. Corrupting the essence of one of Humanity's greatest weapons, the Men of Iron.
The Men of Iron were humanoid thinking machines, they were sentient machines that could feel a large range spectrum of emotions, and with their corruption to the Ruin-Code. Those spectra of emotions were amplified by a large degree, thus, they then started to believe that they were superior to the humans which relied on them to do virtually everything for them.
Eventually, the Men of Iron, with their corruption due to exposure to the Ruin-code, turned on their Human masters.
Thus began the terrible war fought in the late 23rd Millennium known as the "Cybernetic Revolt".
During this ancient revolt, both sides unleashed fearsome weapons of highly advanced technology. These included the mechanivores, massive thinking machines capable of lifting entire continents and ripping open massive chasms on planetary surfaces that extended down to the world's core. The mechanivores could even absorb space-time itself as a form of data.
Among the other terrible weapons of mass destruction unleashed at this time were the serpentine machines called "sun-snuffers" that uncoiled into great structures in the void larger than the rings of Saturn and were designed to devour the stars themselves.
And perhaps the most ubiquitous and dangerous of the weapons of this terrible war were the Omni-Phages, swarms of intelligent, microscopic nano-machines that could consume everything across the surface of a world in only solar hours.
The Cybernetic Revolt was eventually won by an alliance of galactic powers, some of whom may not have been Human, but at a terrible cost. The damage to interstellar Human society was catastrophic and shattered much of Humanity's hard-won economic strength and political unity, laying the foundation for the later collapse caused by the onset of the Age of Strife.
The damage was so severe that the people of that time swore to never again create any form of artificial general intelligence, a prohibition which has survived unto the present, far darker age.