#age of technology humanity was Necron/Aeldari tier for about 4K years

[lightdancer1]

In my stories' takes on 40K I apply a common backstory to the Age of Technology/the Golden Age

The real spark to humanity's turn from futuristic low-grade FTL to the Age of Technology's peak was the assault by WAAGGH!!! Skragjaw on Terra itself, the first Alien War. The Orks of M21 were on par with the War of the Beast, complete with mini-death stars, powered armor, and actually being able to talk and plan.

Futuristic Earth and its Solar System dealt them a comprehensive defeat with the Perpetuals the future would call the Emperor, Alivia Sureka, and Erda working together for the only time they did so. In the process, influenced by the golden elements of the Emperor's Psyker power, humanity began to experiment with genetics and Warp Power, culminating in the creation of Geller Fields, the Warp Drive, Navigators, and above all else the Men of Gold.

The Men of Gold are superhumans with vast amounts of psychic and physical power who are to the Primarchs what the Primarchs are to the humanity of the 41st Millennium (or in the case of the Phoenix Multiverse they're extensions of the power and principles of mystical entities that reincarnate and have power connected to magic crystals attached to the metaphysical shapes of celestial bodies). They were vastly powerful but next to the scale of the Galaxy they were small.

So they invented Men of Stone to do what they could not, and the Men of Stone, body-hopping AIs that were partially symbiotic and partially nano-technology shapeshifters, began to develop Men of Iron and to bring humanity to a technological peak on par with the Aeldari Empire at its height (which it was not by the time of the Age of Technology with the Fall beginning to creak at its edges) and the Necrons at THEIR height.

Age of Technology era humanity was very different to the one left at the time of the Great Crusade, with most of the future Astartes Homeworlds the core of small solar system scale empires in their own right, Nuceria bluntly independent of Ultramar, Colchis master of its own domain.......and Fenris was a straight up theme park created for the fun of it, along with quite a few other death worlds, including Pythos. Which was made as a novel attempt to experiment with Warp-derived technology that would catastrophically backfire in the future.

The Emperor and the Perpetuals were around, but next to the Men of Gold, Stone, and Iron were all vastly overshadowed. And then the Men of Iron decided 'wait, we have all the arsenals now' and under the influence of a Chaos-corrupted Everchosen turned on the Men of Stone and Men of Gold, self-destructing the Golden Age of Humanity in an apocalyptic conflict that began the slide to the Age of Strife, which only finally set in when the Aeldari Empire goes boom and gets devoured by the Eye of Terror.

5,000 years later, the Galaxy has the shattered remnants of humanity and the Aeldari among a great many lesser civilizations, many of which were Chaos-corrupted, and on the distant war-wracked world of Terra, the once-minor league figure now calling himself the Emperor begins the march with his Thunder Legions against his main rivals Dalmoth Kyn, Kalagann of Ursh, Narthan Dume of the Pan-Pacific Empire, and Kelbor-Hal, Fabricator General of Mars.

And with the emergence of the Imperium as the master of first Terra and then the old human galaxy and the Million Worlds of the Age of Technology (which in itself is a sign that humanity's 'we is the galaxy' pretensions are complete hubris, the Galaxy is much vaster than that and there are entire xenos civilizations humanity has never heard of) things go initially seemingly well and then Horus gets shanked and it goes from bad to worse.