There are a few Mysterious Organizations that are well-known, but how much we know about them is extremely limited. There are 2 that show up only during the shifting of an age that claim to be more powerful than most others: the Seers of Io, a group of divine servants of the over-deity Io; and The World-Keepers, a group of wizards that claim to have immense power. There is also a group that has traveled to the Shadowlands and returned.

White Flag
Alliance --> Kingdom of MalitisWyvern AccordUnited IslesDwarven KingdomsElven Empire
Independents --> Church of PelorWizard's AcademyThe Kobold BrigadeTroll Tribes
Guilds --> Thieves GuildAssassins GuildAdventurer's Guild
Cults --> Cult of the DrowCult of NerullCult of C'thulu
Mysterious Organizations --> Seers of IoPact of the ShadowlandsThe World-Keepers