While the Lady of Pain is considered the ultimate ruler of Sigil, the Factions perform virtually all the actual administrative and practical functions of the city. They are the ones the people look to for authority; the Lady only gives edicts through her dabus or appears personally under rare circumstances. Each of the Factions is based around one particular belief system; many of the Factions' beliefs make them enemies where their other goals and actions might have made them allies. All Factions hold many secrets from non-members and even their own members, for the fewer know a secret the more secret it is (and these are secrets of power, either wielded or potentially gained by the Faction's adversaries).

There are fifteen Factions in total, per decree of the Lady of Pain; any additional factions emerging would be subject to her wrath (unless they destroy one of the current 15). At one point there were many more Factions, but the Lady of Pain decreed that they had a week to get the number down to 15 or she would kill them all. Interestingly the Free League membership swelled to over a million, compared to the 20,000 or so members present day.

Factions are led by a Factol. Other high-ranking faction members are called Factors, and mid-level faction members are called Factotums. The lowest-ranking members are called Namers because many of them have only a tenuous faith in their faction's philosophy and are thus members in name only. No one in Sigil has to join a faction, but there is something to be said for safety in numbers.

The fifteen factions are (in alphabetical order):

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