Thoon

Thoon was perhaps a deity or some other entity which was venerated by a specific group of mind flayers, or it could have been a guiding philosophy just as well.

Worshipers
The mind flayers traveled the Underdark, where they spread the philosophy of Thoon. Along the way, they search for a mysterious substance they call quintessence which could be found in creatures as well as in raw materials. Thoon disciples had access to the evil and war domains and wielded heavy flails.

Other "worshipers" of Thoon were the various oddities created by its original followers: shadow flayers, Thoon infiltrators, Thoon thralls, madcrafters, all lead by the Thoon elder brain.

History
Not long after the last arrival of an avatar of Ilsensine in Oryndoll, some mind flayers of the Loretaker Creed went on a pilgrimage to the Cavern of Thought in the Deep Caverns. There was no record if they arrived there or went to some other place, such as the Far Realm. When they came back to Oryndoll, they proclaimed the belief in Thoon - a doctrine for which they were outcast by the clergy of Ilsensine.