Mauve psionic brain-eating race, once rulers of an interplanar empire
In the past, distant beyond comprehension, the illithids held sway over a titanic planar empire, which reached from the Astral to the Ethereal, and included countless Prime worlds. The mind flayers were vastly powerful, and caused even the great armies of the Outer Planes to give pause. The slaves held by these illithids were used for physical labor, livestock for feeding, and cruel experiements. Over these hapless prisoners, the mind flayers ruled supreme.
Slave revolts came and went, but none could truly challenge the authority of the mind flayers, whose awesome mental powers were far beyond the capabilities of their malnourished prey. Eventually, though, one woman named Gith came forward. She had been trained in warfare and knew the ways of the illithids. She was strong in body, will, and mind, and led a great slave rebellion. Her armies overtook the mind flayers and brought down their empire.
The illithids may have been exterminated at this point, if not for the protests of one of Gith's followers, a man of growing influence among "the People," as they were now called. Zerthimon, as the man was named, told the People that Gith was an evil leader and would lead them down the same road as the illithids had. A great civil war ensued, which left Zerthimon dead and Gith with not enough warriors to complete her bloody mission of vengeance. The two factions split and became the githzerai and the githyanki.
Meanwhile, however, the illithids escaped. They now plot in secret, no longer possessing a grand empire, but maintaining their vile cunning, their insidious greed, and their sadistic contempt for all "lesser" races. An illithid uses its massive psionic power to shred its enemies' minds at range before devouring its brain with revolting satisfaction. Its form is equally hideous: a roughly humanoid body with wicked claws, rubbery skin, and an alien head featuring deep empty eyes and four tentacles around a beaklike mouth. They work in hiding among the most remote reaches of the multiverse to bring down their enemies, and to secure more power for their self-serving and cruel agendas.