Demarch's Alliance was an alliance between beguilers who served Leira and those who served Mask. But after Mask betrayed Leira in the Year of Shadows, 1358 DR, the alliance was broken and the two factions fought a massive battle in their former headquarters, the Halls of Demarch.
The rival beguilers fought a number of bloody skirmishes through the tunnels of the subterranean complex, all concealed in mists. An enormous amount of magic was cast, resulting in the opening of a rift to the Plane of Shadow. Shadows and various other monsters fell out of the rift, along with several magic items and the book of Suelivain. The Maskarran and Leiran beguilers slaughtered eachother.
The survivors fled across Faerûn, living in fear of revenge attacks and church agents seeking to learn or maintain their secrets, most never coming back. A few friendly ex-members, however, returned to the Halls of Demarch and reformed into the Demarch's Folly organization.