From Introduction:
Nobody knows when the illithids first appeared in this world and some claim that their existence even predates that of all other races. The first recorded encounter was 500 years ago, when the mind flayers rose from the sea in their kraken-like submarines and started taking slaves among the coastal population. Ever since then the flayers where a looming threat, pirates and raiders with no major political impact other than limiting trade with races from outside the Empire.
It was during the civil war that theses foul creatures rose to major importance. The civil war increased the connection between the real world and the Realms Beyond. For the first time, the Wheel of Law, the mind flayers foul god, was able to give direct orders to his followers, forcing them to ally with his other servants, such as the Red-Gloved Daimyo. Ever since, the illithids are a power to be reckoned with and everybody expects them to invade the surface world any time.