Thuria and Beyond
Based on the basic AD&D 1E stats and the rich D&D literature about this species, ilithids in our campaign reside in Underdark settlements far below the surface of the earth.
The ilithid species came into being around 18,000 YT, in the dying days of the War of the Oceans, a titanic conflict between the aboleth of the surface oceans and the Star Spawn of Great Cthulhu. The majority of the once numerous Star Spawn perished in this terrible orgy of destruction and a good number were sealed, slumbering in the city of R’lyeh along with their master.
The few Star Spawn who sought escape from the destruction in the oceans by wading ashore find dry land to be inhospitable. These survivors adapted to dry land conditions by modifying their means of reproduction – they produced larvae which could be implanted into the brains of sapient creatures. By this means, the first ilithids were created. Not yet numerous enough to overwhelm human and humanoid tribes, the ilithids retreated to the Underdark, where they began to establish major settlements and develop a distinct civilisation of their own.