In the pre-history of the cosmos horrible aberrations from beyond the stars slipped into our world through cracks in reality. They were banished by or destroyed by the forces of the Old Ones, but some of their greatest champions and servitors remain imprisoned or hidden within the cosmos. From these followers other races have descended or been created.
They are from the blackest depths of insanity from between the stars, the cold vacuum of misaligned space and time, and the darkness between worlds and our inner minds!
Mind Flayers seem to have no creation or place of origin. All that is known is that they once ruled a cosmos spanning empire, were overthrown by a slave race, and plot from the shadows to recreate their star spanning empire.
Illithids venerate an unimaginable evil imprisoned and slumbering within the Astral Sea. This being named Cthulhu bears a great resemblance to a large six-tentacled Mind Flayer with large bat like wings and a greenish coloration.
Mind Flayers are at odds with many other aberrational races, for they have no wish to destroy the cosmos, they “merely” seek to bring darkness and domination to the world in preparation for the rise of Cthulhu’s city of Ry’leh from the depths of the Astral Sea.
Aboleths are perhaps the oldest intelligent race in the cosmos. They are descended from the ancient demon lord Dagon. Aboleths do not worship Dagon as a god, since their race predates gods, but venerate it for its philosophy and oracular powers.
With their inherent flesh-shaping powers, Aboleths have created many slave races and their other experiments have created horrendous and unique monsters. The race known as Deep Ones (Kuo-Toa) are their primary servitor race, and they created the sahuagin breed as a race of warriors. One of the Aboleth’s favorite races to experiment on are humans, and they are continuously attempting to create the perfect aquatic/human hybrid.