It should be noted that the story of the Illithids is circular. In the far future, Mind-Flayers had conquered the universe. However, the end of times is here. To not die some of them travel back in time to the beginning and start to conquer everything all over again. They either bring with them or enslave the Gith. Gith revolts and destroys the growing Illithid empire. One of the emerging factions of Gith becomes a new imperialist domain hell-bent on conquering the entire universe.
Now, who are the Illithids? They used to be something else before they became squid headed psionic powerhouses. A common theory is that they were humans. I prefer the theory that they were Gith.
So you have a story where Illithids travel back in time to escape the end of the universe. They enslave (or bring with them) their Gith slaves. The Gith revolts and crushes the Mind-flayer empire. The Gith become the Githyanki and start conquering the universe. Their quest to master their psionic powers and enslave the universe eventually corrupts them into Illithids -- the Gith who don't ascend become their slaves. At the end of time, they travel back in time to escape the end of the universe.
