Old Ones
The Old Ones were an extremely advanced and powerful tech-sorcerer race of beings back when the grass was green and the Emperor wasn't born yet.
Then they met the Necrontyr, and all hell broke loose.
The Necrontyr, being miserable piles of shit with miserable short lives envied the Old Ones and started to war against them.
The Old Ones fucked them seven ways 'till Sunday until the Necrontyr, who realised they were not strong enough to win, discovered the Star-Gods, the C'tan. Blah blah sacrifice our souls and live as cold metal automatons blah blah fight the Old ones.
The C'tans said,"No Necrontyr,YOU ARE THE NECRONS". And Necron was a grimdark.
At this point the Old Ones supposedly created other races to row row fight the Necrons: The Eldar, the Orks and the Jokaero, but thought humans should just be made evolved a little, therefore psykers being rare. That's right, mutant psykers suffer because they are marginalised and thus is the Imperium suffering. Well done Old Ones.
All the fighting made the then-silent Warp as it is in a copy of Catholic Hell, with all FIGHTAN and FAIL filled in it.
In the outcome, the Necrons and their C'Tan overlords were forced(?) into hibernation when Enslavers, Krell and Illithid rip-offs came out of another dimension and buttfucked every last Old One and 99% of their minions, and so made Necrons go hungry, and the new and young races survived when every last Necron went to sleep.
The Old Ones themselves, seeming to have done one Acid strip too many, promptly disappeared from the scene,except playing the video game Spore. Fuck off you shitcunts, we owe you all the grimdark and fail.