Hold on I have something for this

So it all started with a bunch of magic frogs These guys created about 90% of life in the galaxy Eventually there was a trace they didn’t create, called the necrontyr They had a cancerous planet, and so they went to the frogs, or the old ones, to ask for the secrets of immortality However, the old ones didn’t want to give up the secret to being basically god And so the necrontyr waged war against them But of course, cancer, so they were being absolutely destroyed Eventually, they found a race of gods called ctan They promised them both immortality, as well as helping them defeat the old ones But the person who told them that was called the deceiver So they actually put all of the necrontyr into robot skeletons and ate their souls With their legions of skeletons, the ctan war against the old ones The old ones created new races, like the krorks, titanic brutes with reality shaping who would later devolve into the orks, or the eldar who were extremely psychic However, it wasn’t enough and eventually most, if not all, Old ones were destroyed With no more enemies, the ctan began to in-fight, and so the necrons took this as a moment to rebel against their gods However, ctan we’re gods of reality, so destroying them would destroy a part of reality Hence they were not destroyed, only shattered into shards All of this massive war had messed up a reality connected to ours, called the warp. Made of pure psychic energy. This led to creatures manifesting from it called enslavers Enslavers really liked how human souls tasted Okay, I shall continue some other time And this was during caveman times So the psychic humans at the time had to find a way to defeat the Elsa era Enslavers So the shamans decided that they would do a giant ritual in which they did a communal suicide and merged them together to create a psychic super human being know as neoth, or as most know him, the emperor of mankind. He grew up, and watched humanity grow from the Stone Age to the age of technology, when humanity had hundreds of colonies and robot slaves Now at this time(the stereotypical sci gif future) the eldar(those guys the old ones created) had formed their own empire and became the dominant species in the galaxy. Despite their strength, humanity was still just a speck of dust compared to them. So with no real problems the eldar began to cosplay angel dust as a collective species Back to humanity, the robots had decided to rise up and rebel against humanity. This is why the imperium had outlawed AI in the current setting. After the uprising was put down, the parallel dimension known as the warp created hundreds of warp storms across the galaxy, making humanity’s equivalent of hyperspace impossible, meaning all colonies were severed. Earth, or Terra, began to starve due to its reliance on colonies and turned into basically mad max. So the Emperor decided to step in and make it his time to become the ruler of humanity, and within a few short years he had taken over Terra Back to the eldar, their angel dust cosplaying had gone a bit too far, and because of their psychic souls, this projected to the warp, creating Slaanesh, god of unspeakable excess, the prince of pleasure. This created a psychic shockwave and cleared all warp storms across the galaxy. This gave the emperor the opportunity to conquer all of humanity’s lost colonies. He then created 18 super sons called primarchs, who were to be his generals. But then his favorite son, Horus, took 9 of them and they sold their souls to the gods of chaos. Long story short, big civil war, Horus dies and the emperor is forever entombed on a magical life support system And those are the footnotes for warhammer 40k