The Twisted Rune
is a secret organization of undead spellcasters, operating in southern Faerûn
It is possible to have battle with several of their members in Shadows of Amn, if you bring a rogue stone to the portal in the Bridge District, you will be teleported to their hideout and fight them.
Balthazar's mercenaries rumored that he had been co-operating with members of this organization. See Twisted Rune Building.