The Citadel of Eight
The Citadel of Eight was formed at some point after 561 CY, and was dissolved in 569 CY. Its initial membership was Mordenkainen (its founder) and Bigby, his apprentice. The two decided to form a group to attempt to maintain the balance between the extremes of Oerth. They recruited Robilar, Riggby, Yrag, Tenser, Serten, and Otis. The name of the group was based on their number and Mordenkainen’s Obsidian Citadel.
The group eventually dissolved for ideological reasons (in the case of Robilar and Otis), because of grudges between members (such as Tenser who blamed Mordenkainen for the death of Serten), or because of outright death in the case of Serten who fell in 569 CY fighting against the hordes of evil