In ancient times, The Illith were a race of Philosopher kings with great mental powers, who sought nothing more than understanding and inner peace.
They spent most of their time meditating, communicating only telepathically, and scrying on the world. In their scrying they saw the other races, but the other races had nothing that the Illith wanted, save for technology. Technology was easily copied through scrying, however. The Illith derived a form of life magic from that of the Sidhe, and used it to turn crabs into superdense suits. In these suits, many made pilgrimages to the bottom of the Great Trench, to meditate in an area of natural beauty.
Then came the Great Fuckup of the Goblins.
A continent laden with Lich Kings and their innumerable servants came crashing down. Shockwaves and turbidity alone killed hundreds of thousands. The undead creatures did not care for the Illith, and they stood between the main Illith settlement and the Great Trench.
War erupted. The crab suits previously used only to explore the depths were redesigned for combat. They amplified the Illithid strength and blades bounced off their hardened chitin. Sea snails were turned into daggers, and then swords. Mantis shrimp were turned into instruments of pure concussive destruction. Urchins were modified to launch their spines. Everything had advanced poisons.
Gradually the Illith got the upper hand, and while not able to eliminate the goblin liches, were able to achieve a victory. The Great Trench was eventually reunited with the rest of Illithid society, and the Undead began to avoid the Illith.
Using their newfound freedom the Illith have begun investigating the surface world which had visited such horrors on them, and they found it wanting. Now, equipped with the war-gear they have developed over centuries, the Illith jihad, or Illithiad, makes war. The surface world so far is ignorant, seeing only missing ships or vanished fishing villages. And who believes a few Furnshakt stories, anyway? But when the time is right the surface world will tremble at the coming of the Illithiad.