The Planewalker's Guild is an old guild, dating back to before the time the Great Upheaval. The guild is newly returned to Sigil, having spent nearly a millenium based on the Infinite Staircase.
Its a guild that believes that only travelling the planes can reveal the true nature of the Multiverse.
Their guild-master is Balthasar Thames the Knowledgeable and their headquarters is in the Market Ward of Sigil.
Heh, well if you thought THAT was impressive just wait 'till you see the water clock!
— Tinker gnome at the annual Ruby Goldenburg inventors' contest. This cycle's winner: A Better Wererat Trap