Factions

Factions in Planescape are important!

List of Factions
Athar

Believers of the Source

Bleak Cabal

Doomguard

Dustmen

Fated

Fraternity of Order

Free League

Harmonium

Mercykillers

Revolutionary League

Sign of One

Society of Sensation

Transcendent Order

Xaositects

The Coalition of the Banished

The Church of Humanity Ascendant

Observers

The Corporation

The Illustrious Company of Enforcement and Pursuit

The Combined Ring Planes Militant

Details
You can make your own faction! You have to have a philosophy to begin recruiting. The real benefit is you can gain a prestige class and make your own.

Each faction listed also has 1-5 feats, and a unique prestige class for them, typically an improved version of an existing presitge class