Mentioned briefly in A Study of the Progeny of Tezirah.
When the Saesordian Cabal was founded in 718BL by Tezirah, they were the forerunners to the Progeny in the most literal sense, not only discussing the theories behind shadow magic, illusion, and sorcery, but the actual practice. Sorcery was a socially unacceptable and taboo practice but was legal until the Empire passed laws banning it 12 years after the group was founded. The banning of sorcery coincided with the order to execute Tezirah and the entire group went underground for three years.
When Tezirah was executed the group dissolved for nearly a century (79 years). Heronyus Kalestraum staged a reformation of the group at this time and it existed after this reformation as the Saesordian Cabal headed by Heronyus, including the Crowther family, for twelve years.
The stated reason in books for the Crowthers' split from the Cabal was that Kalestraum was more interested in the practice and development of sorcery rather than actual philosophical discussion and magic that was actually legal at the time (practicing sorcerers still being burnt at the stake). What ulterior motives and desires for power on the part of the Crowthers is unknown, but the Crowthers split from the main group all the same, forming the Progeny of Tezirah.
Right after this schism happens and the Progeny forms, the Celestians begin to see Kalestraum and the Saesordians as a threat to their fledgling group and their desires for legitimacy in the Empire. The event we know as the Scouring takes place, lead by someone known only as the Eye. In plainer terms, the Compact went to war against the Cabal using unknown but apparently experimental group-based magic and won, slaughtering most of the group in the process. The remnants of the group, less Heronyus who was allegedly killed in the battle, go underground, not to resurface for centuries.