The Battle of Demarch's Fall was a battle fought in the Halls of Demarch beneath the city of Tantras in the Vast between rival factions of Demarch's Alliance, an alliance between beguilers who served Leira and those who served Mask.
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 tome of Suelivain. The Maskarran and Leiran beguilers slaughtered each other.
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.