Ilthura are also called "Progenitors", "Brain-Benders", and "Mind-Flayers" by the other cultures of Asmythe.
The Ilthura are a strange race that controlled an empire spanning all across Asmythe, but they haven't been seen since the end of the First Era. Not much is known about them. There was no spoken language when the Ilthura disappeared and Ilthura writings are more likely to cause insanity than understanding.
Nobody knows where they went or why they disappeared, but some Dragon Priests claim the third moon has something to do with it.