Era of the Illithid

Summary
After a well though out plan of attack, the Illithid took over our world from the Astral Plane. This rule would have gone on indefinitely if the Gith races hadn't started a revolution against their slave masters and toppled their empire that crossed universes.

30,000 P.C. - 27,000 P.C.
The Illithid moved quickly across the world, killing anyone who could prove to be a threat, which nearly wiped out the Giant race. Many of them hid and retreated, surviving in their caves up in the mountains. The Illithid proved brilliant strategists and took over quickly.

What followed was terrible. Tribes and settlements were manipulated, tortured, and even worse, sometimes eaten by their psionic overlords. They couldn't do anything about it, as people of this world had no means to fight against their mind control or their overwhelming military might from the Gith.

27,000 P.C. - 24,000 P.C.
Eventually the world fell into complacency. The Illithid, spreading their influence over more worlds and more planes, eventually began to spend less and less time on this world. The horror of living under the thumb of these creatures eventually began to lessen. The Gith even began to bond with the races they were in charge of keeping in line.

The Gith couldn't be too nice or get too friendly, though, as their masters would kill them as well. So, they became stealthy and clever about their actions, knowing that any casual reading of their mind could give away their subtle rebellion to the Illithid. But they waited and bided their time.

24,000 P.C. - 23,000 P.C.
Eventually, aligned with the natives of this world and on numerous worlds across the multiverse, a man named Gith (the namesake of the race) led a rebellion against the mind flayers. The war lasted centuries and costs many lives, but eventually the Illithid had to leave this world and retreat to their own world and regroup...

Signs of the Era
Mind Flayers, or Illithids, still exist in this world, but they have driven deep underground. Various creatures like the Intellect Devourers, Grimlocks, Duergar, and Kuo-Toa remain behind as a reminder of the occupation of their kind. Also, after the rebellion, various factions of the Gith, eventually known as the Githyanki and Githzerai, either got left behind or stayed behind on this world.

Various magical artifacts and items have been left behind as well. Coming from otherworldly places, these magic items had been brought here by the Illithid and their slaves long ago and were left behind when they fled. Some have survived to this day, and their purpose or abilities as a varied as the multiverse itself.