The Halls of Demarch was a secret guildhall, hidden beneath the city of Tantras. It consisted of a subterranean complex and a network of tunnels.
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 Battle of Demarch's Fall. The rival beguilers fought a number of bloody skirmishes through the tunnels, 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 each other.
The survivors fled across Faerûn, living in fear of revenge attacks and church agents seeking to learn or maintain their secrets. A few friendly ex-members, however, returned to the Halls of Demarch, believing the safest place to hide was the last place they were expected. They reformed into a new organization, taking the name Demarch's Folly, and learned shadow magic from the book of Suelivain.