The Sign of One is one of the Factions in Sigil. Their philosophy is that all of the multiverse is just an illusion dreamed up by the Self, and all locations and people are simply reflections of the Self, much like dream images reflect the thoughts of the dreamer. Most Signers believe that they are actually the one Self, and all the rest of reality is their delusion. Other Signers believe that there are multiple Selves, and each dream reality overlaps with others. Regardless of which variation they believe in, members of the Sign of One are largely solipsists who focus on themselves above others.
The Sign of One runs the Hall of Speakers, which is the meeting place of all the official Faction representatives in Sigil. Here each Faction debates the finer points of city laws and regulations, hashing out rules for Sigil that most of the Factions more or less agree on most of the time. Nobody is ever truly happy with the eternal compromises, but the Sign of One is happy to facilitate… especially when it gives them considerable power to dictate the terms of these debates.
Faction Abilities and Drawbacks
Because Signers believe all reality is an illusion of their own creation, they're very sensitive to the illusions of others. When confronting illusion magic, Signers may automatically roll to detect it (usually the GM will roll secretly and inform the PC when they succeed).
The Sign of One is very self-centered, however, and their egotistical attitude rubs many the wrong way. When encountering an NPC, their starting attitude towards Signer characters is automatically one category lower than it otherwise would be (Friendly to Indifferent, Indifferent to Unfriendly, etc). Other circumstances can nullify this effect as the GM dictates, including the PCs personal reputation, other friendly affiliations, etc.