Era of Blight

Summary
Left to rot in the wasteland the Illithid left behind, the people of this world suffered. Famine, disease, and death followed the Illithid leaving this world. Many of the Gith left the world and its inhabitants to pick up the mess, and they endured the chaos for almost a thousand years.

23,000 P.C. - 22,200 P.C.
Many people tried to pick up the pieces left behind after the Gith rebelled against the Illithid. Some of the previous leaders of this world (Dragons and Giants) even made a play to retake the world, but neither of them were strong enough and failed numerous times.

A few magical secrets and items were left behind by the Illithid, and various natives of this world tried to use them to escape and leave this dying world behind. Unfortunately, it didn't get them off this world, but ended up bringing someone here.

22,200 P.C. - 22,000 P.C.
The Efreet, coming from the elemental planes, came under the guise of granting the people wishes. However, once they were in the world, they summoned others, and others, and others, and eventually the Efreet decided to take this world for themselves...

Signs of the Era
Many of the secrets, spells, or magical items that are used for travel between worlds were discovered around this time. Much of the knowledge used to form a comprehensive map of the planes was discovered during this time as well, obviously at great cost.