Age of Strife
The Age of Strife, also known as Old Night, is a period of the history of Warhammer 40,000, and in particular the Imperium of Man.
After figuring out the Warp Drive and the Navigator gene in the late M1X's, humanity was able to spread out from the solar system to other stars, and with the STC system, they could make anything they needed no matter where they landed. Their military was augmented by robotic warriors known as the Men of Iron.
Unfortunately, all that travel and commerce depended on the Warp being navigable. During the M20's to M25's, the Warp started getting fouled up by Daemons and storms as humanity's psyker population increased. Since the Astronomican wasn't due to be built for another few thousand years, nobody had any way to get anywhere, and so humanity's star empire fell apart. Without its colonies to supply it, order broke down on Earth as the various factions turned on each other. To make matters worse, the Men of Iron rebelled and threw Earth into a further state of anarchy before they were finally destroyed.
Things would remain grim and dark into the late M29's and early M30's. At that time, the depravities of the Eldar Empire (which had furthered the turmoil in the Warp) reached their climax, resulting in the birth of Slaanesh. This released a lot of the pent-up energy of the Warp, allowing the storms to die down, and that's when the Emperor of Mankind chose to make his entrance. He immediately set out to re-unify Terra, and once he had done that, he launched a Great Crusade to gather the lost colonies of humanity into a mighty Imperium of Man.
What the Emperor was doing while everything fell to pieces is anyone's guess. If you believe a really old theory that doesn't get any attention anymore, being pushed out of a vagina somewhere in the mid-M2X's.