The Imprisoners were a group of powerful mages working before the Spellplague for Larloch who imprisoned spirits in magic items. The ultimate goal was apparently to preserve some of Larloch's power so he could access it later. The items created were known as Blueflame items and had powerful magic contained within.
Mystra's clergy became aware of the plot but the goddess refused to allow any interference. The Simbul believed the Blueflame items created by the Imprisoners contained enough of Mystra's power to restore the goddess.