Thayan Resurrection

The Thayan Resurrection was a secret rebel organization born among the Red Wizards of Thay, composed of wizards of all magic schools minus Necromancy with the objective of overthrow Szass Tam and the Necromancers from the rule of Thay. Among the group were sincere revolutionary hoping for a land more adapt to human life and power-hungers that want replace a ruler with an other, usually themselves. They were structured in autonomous cells of few members in oder to made difficult for Tam's followers destroy them with a single blow. Two noted members were Syranna and menek ariz that with adventurers' help destroy the most loyal Liches of Tam in the secret dungeon of Doomvault.

Behind the scenes
It's possible that one of the founders of the Thayan Resurrection was Umara Ankhlab that at the end of The Reaver (novel) abandoned the nocromacer's path hoping for a resurrection of Thay