Sign of One
The Sign of One, also known as the Signers, are a Faction in Planescape defined by their belief in the sheer power of belief.
See, the idea that what you believe can become reality is pretty essential to Planescape's underlying philosophical themes. But the Signers take this to the ultimate conclusion; essentially a Faction based on solipsism, Signers believe that if they believe anything hard enough, they can make it happen right here, right now. They are, literally, advocates for the power of thought to shape reality, and the value of positive thinking. In a nutshell, the Signers believe themselves to literally control the multiverse.
Incidentally, this gives them a bad relationship with the Transcendent Order and the Bleak Cabal, both of whom find their philosophy offensive, if for different reasons - the Ciphers hate the Signers' assertation that thought is the power that controls the cosmos, whilst the idea that people ultimately shape the multiverse around them through their thoughts is inherently opposed to the Bleakers' views that the planes are meaningless, random and pointless.