system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] Is there any TL;DR about history?
gpt
Here we go...

At the dawn of time, there were beings called the C'Tan who ate stars.

Then the Old Ones arose. The lived long, pleasant lives.

Then the Necrontyr arose. The Necrontyr lived on a harsh planet orbiting a crap star; as such, they were short and didn't live very long. Long ago, they had an empire that went to war with the Old Ones, moslty over jealousy that the Old Ones lived such happier lives. The C'Tan at this point began eating souls instead of stars, and since the Old Ones' souls were particularly tasty, they allied with the Necontyr to defeat the Old Ones. This entailed the Necrontyr putting their minds into robotic bodies, but as they did so, the C'Tan ate their souls, leaving the Necrons as immortal but mindless slaves, now called the Necrons.

The Necrons, backed by the C'Tan, gained the upper hand and succeeded in wiping out the Old Ones, but not before the Old Ones uplifted the Eldar and created the Orkz. By the end of the war, there were only four C'Tan left - the Deceiver, Outsider, Night Bringer, and Void Dragon, the rest having been destroyed either by the Old Ones and Eldar (original continuity) or a rebellion by the Necrons themselves (current continuity). The Necrons, exhausted by the war against the Old Ones and unable to stand up to the rise of the Eldar, went into hibernation.

Eldar came to rule the galaxy, and for about 60 million years nothing much happened.

In 8000 BCE, the shamans of Earth came together and committed mass suicide so that all their souls would be reborn in a single person, the Emperor.

Eldar and human emotions give rise to the Chaos gods Tzeentch, Nurgle, and Khorne.

Eldar achieved a post-scarcity society which used robots to do all its fighting. A few think this is a bad idea and retreat to more ascetic planets on the galactic rim, becoming the Exodite Eldar. The rest live lives of pointless excess and hedonism, and don't notice humans gradually carving out a new empire, starting in the 18th millennium.

Around the 23rd millennium, psykers began to emerge among humanity, and brought with them daemoic possession. Human worlds collapsed into anarchy, and Earth was surrounded by Warp storms. Civilisation completely collapsed on Earth - think *Mad Max* but one a bigger scale with better guns and more grimdark. Around the 27th millennium, the Emperor managed to unite Earth's technobarbarians into a single polity. and also gained the support and technology of the Martian government.

In the 28th millennium, the Eldar's collective hedonism finally came to a head as all their selfish energies combined to form Slaanesh, which ripped open the Eye of Terror around the Eldar homeworld and, as a side effect, dispelled the warp storms surrounding Earth. Some of the remaining Eldar decided to live on nomadic ships and hope to one day combine their souls to form Ynnead, the new god of death, who will be able to kill Slaanesh. The rest, the Dark Eldar, live in a city in the webway where they indulge in ecstasy and insane torture of other beings in order to keep Slaanesh from eating their own souls. Meanwhile, the Emperor managed to reconquer planets of the old human empire.

In the late 30th millennium, the Emperor created the Primarchs by genetic engineering as the first of what would eventually become the Space Marines. The Chaos Gods, fearful of what the Emperor would bring, interfered somehow and scattered the baby Primarchs across the galaxy (though all managed to land on human planets). The Emperor launched a massive crusade to restore the primarchs and reunite humanity, which he did.

Then in the early 31st millennium, Horus led a rebellion against the Emperor, brining half the Space Marines and a full third of the Imperial Guard with him. He was narrowly defeated, but the Emperor was critically wounded, and thus was interred on the Golden Throne to watch over and guide humanity. Remaining Traitor Legions retreat into the Eye of Terror. Roboute Guilliman instituted a reform of the Space Marines into a large number of smaller chapters, though not everybody approved, and Leman Russ flat-out refused to accept the new rules because of course he did. The surviving loyalist Primarchs gradually disappeared.

In the 36th millennium, the Ethereal Caste united Tau (who might possibly be surviving Necrontyr) society under the philosophy of the Greater Good. Shortly therafter, the Tau discovered a spaceship and successfully reverse engineered it to give limited Warp travel.

Around the 41st millennium, Tyranids appeared in the galaxy (though there is evidence they had a small presence as early at the 35th millennium) and the Necrons began to reawaken.

And thus in the grim darkness of the present time, there is only war.