A faction is an organized group of like-minded individuals built around a core philosophical belief. There are fifteen factions in the Planescape setting, though only five can be joined by The Nameless One. According to Able Ponder-Thought, there were once forty-nine factions in Sigil. At some point, the Lady of Pain declared that only fifteen should remain. Shortly thereafter, everyone who was not a member of one of the chosen factions wound up dead. This event came to be known as the Great Upheaval.