Demarch's Folly was a spellcasting organization based in the Halls of Demarch in the city of Tantras in the Vast in north Faerûn. Its members were dedicated to learning shadow magic and other shadow-based magics, such as the Shadow Weave.
Originally, Demarch's Alliance was a group of beguilers devoted to Leira and Mask. But shortly after the Time of Troubles of the Year of Shadows, 1358 DR, Mask betrayed Leira, and Demarch's Alliance was broken. The two factions fought the vicious Battle of Demarch's Fall in the Halls of Demarch, leaving many dead and their former base infested with undead, shrouded in mist, and the site of a rift to the Plane of Shadow.
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, believing the safest place to hide was the last place they were expected. They took the name Demarch's Folly.
They also sought a more solid and direct form of magical power than illusions for their protection. Ironically, they found it in their very own halls. The rift to the Plane of Shadow had deposited a number of magic items and the tome of Suelivain. From the tome, they learned the fundamentals of shadow magic. Later, they began seeking out beings mentioned in the tome and finding other works about shadow magic, and dedicating themselves and Demarch's Folly to its mastery.
They tracked down Suelivain, a shade of the City of Shade and a shadowcaster, and returned his tome and other possessions in exchange for training in the ways of shadow magic. Suelivain accepted and joined Demarch's Folly in the role of mentor and leader. He educated them in the differences between shadow magic and Shadow Weave magic.
Demarch's Folly was an arcane guild. It consisted of and accepted new members who had some shadow-based powers, such as shadowcasters, users of Shadow Weave magic, shadowcrafters, and shadowdancers, or were of races innately connected to shadow, like shades, illumians, and half–shadow dragons. Members were also required to have a good deal of knowledge about the arcane arts, religion, the planes, and/or history.
Members were required to pay dues to Demarch's Folly, equal to a 10% tithe of their earnings.
The members of Demarch's Folly journeyed the world in search of information about shadow magic and all forms of shadow powers. They sought fragments of knowledge, people who knew about these things, and relics or magic items. They offered their services in exchange for information and training, and usually dealt fairly with those who had what they needed. Although a number of its members were evil, they only occasionally resorted to theft or intimidation.