Age of Strife.

The age of Strife began in the year 20K and ended in the year 25K. (I'm seeing a pattern hear.) Humanity had lost access to most of its technology, space travel and the ability to contact human's on other planets. Human's on planets that couldn't grow their own food didn't last long.

The wave of warp energy created by Slaaneshs birthed the first human psykers that aren't Perpetual's or Navigators. Planets that stayed progressive was happy to have them. Planets that regressed treated them like witches and wiped them out. Unfortunately these psykers did not know how to control their power's. So their brains became portals demon's could escape the warp from and corrupt the people. The regressed planets didn't have this issue.

Even if a planet could grow their own food and didn't have psykers, they weren't safe either. Aliens that where at war with humanity and aliens that where once their allies attacked them to steal their reasorces.

The Emperor of Man Kind had been around during the cave man era. However he didn't want to rule over humanity. So instead he was a silent guardian, not wanting humanity to even know he existed. That changed when the age of Strife began. He knew if he didn't take control, man kind would go extinct.

He took 5 000 years tracking down his fellow perpetuals that had spread across the galaxy and convinced them to join his cause of saving humanity. Unfortunately they realised he was a violent psychopath and wanted no part in his vision. Only 2 perpetuals stayed by his side.

He then created the Custodes and Thunder Warriors. Concord Terra before the population could destroy themselves. Created the Primarchs and the space marines. Was unable to stop his lover from launching the Primarchs into space. Got fused with a Chaos goddess. Convinced her lover to stay with her and started a crusade to reclaim the lost colony worlds and find the Primarchs.

Even though the age of Strife was officially over, many colony's continued to suffer as if the age of Stife was still happening for another 5 000 years. The age of Strife effects has scarded humanity, making them the evil monsters we know in the year 30k through 40k. The Empress of Pleasure continues to work on not only keeping them safe, but to make humanity kind once more.