The Circle of Eight
The Circle of Eight are a powerful group of wizards with an unknown agenda. They appear frequently in the city of Greyhawk, but Mordenkainen has a fortress called the Obsidian Citadel in the Yatil Mountains. They are generally assumed to control the city of Greyhawk.
Seven of the Circle of Eight are known to the public.
Membership in the Circle of Eight is the ultimate goal for many wizards. So far, the organization is new enough that no members have left. Since they’re wizards, it might be a while before any openings come up.