A Great Conclave is a forum that may be called in Thurkad during times of grave crisis. On occasion, a Conclave throws up a new way or even a new leader. A Conclave is held in the city itself, rather than the Magister's Citadel.
In 0 BTM, Mendark, as former Magister, called a Great Conclave so as to force Thyllan to submit the Mirror of Aachan to the Just, in the hopes he would be granted ownership of it. This particular Conclave was violated by Tensor, who used a mind-blasting potency to kill the Prime Just, Nelissa, and lay the whole Conclave low, before fleeing with the Mirror.