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human
[Warhammer 40K] When did Earth and the Galaxy start to go to hell?

It followed our timeline (roughly) surely the universe wasn't always this bad.
gpt
There's 2 events that mark the galaxy's fall to GRIMMDARKNESS, the War in Heaven, and the creation of Slaanesh.

It all started about 60 million years ago. At the time there were 2 species of note in the galaxy; the Old Ones, a race of psychic toadmen, and the Necrontyr, the precursors of the Necrons. The Old Ones had found a way to transcend their physical bodies and spent most of their time in the Realm of Souls, a psychic dimension created by the collective subconscious of sentient races. You could say that they had achieved the pinnacle of evolution. The Necrontyr were super jealous of the Old Ones power, so they attacked them. Obviously this was a terrible idea, so in desperation they took the C'tan, a bunch of space clouds that fed on radiation, and gave them physical bodies hoping to make superweapons. The C'tan developed a taste for souls and enslaved the Necrontyr, forging metallic bodies for them.  With the C'tan's power the Necrontyr, now the Necrons, were finally a threat to the Old Ones. Terrified of these massive soul sucking monstrosities the Necrontyr had unleashed, the Old Ones created their own monsters, the Orks. Around this point one of the C'tan named the Deceiver convinced the most powerful C'tan to start eating their weaker brethren to become more powerful. The resulting infighting weakened the C'tan so much that the Orks overwhelmed their empire. Those that remained went into hibernation hoping to wait out their enemies. With the C'tan and Necrons gone, the Orks turned their attention to the Old Ones and finished off most of those that remained. All of this destruction turned the Realm of Souls into a terrifying nightmare called the Warp.

At this time mammals were beginning to dominate earth.

The creation of the Warp wasn't great, but things weren't too bad until Slaanesh was created. With all of the most powerful races out of the picture, the Eldar began to dominate the galaxy. They had a long golden age, but unfortunately life started to get boring. They slowly became more hedonistic, indulging in some really fucked up shit. Eventually their obsession with sensual pleasures created a new Chaos god, Slaanesh. Slaanesh's birth completely destroyed the Eldar Empire, the center of which became overrun with warp energy creating the Eye of Terror. Only the Eldar living at the edge of their civilization and in the Webway were spared. This flood of chaotic energy completely threw off any semblance of balance the galaxy had left.

At this time humanity had developed its own interstellar civilization. Slaanesh's creation caused psykers to appear in massive numbers, beginning the Age of Strife. Humanity was plunged into a dark age until the Emperor appeared and went about uniting what was left of humanity.