The Ilthids are a strange race that lives deep underground in depths so deep they are called the "underdark". The ilthids shun light, greatest punishment an ilthid can receive is to be banished to the surface. The ilthids are believed to be an ancient race who was building empires when the other races were still struggling to walk. Their connection to the Forge is surpassed only by the eternals.
Ilthids are also called "Progenitors", "Brain-Benders", and "Mind-Flayers" by the other civilized races. Where goblins are brutal barbarians of the surface world the ilthids are the cunning overloads of the underground. The ilthids seem to regard the other civilized races with contempt, much like we would view a mangy mut, and kept them as slaves and livestock.
Most of the ilthids disappeared from the world at the end of the second age. Nobody knows where they went or why, but some sages think the appearance of the third moon has something to do with it.